diff --git a/src/collection/album.rs b/src/collection/album.rs
new file mode 100644
index 0000000..40da9a1
--- /dev/null
+++ b/src/collection/album.rs
@@ -0,0 +1,42 @@
+use core::mem;
+
+use serde::{Deserialize, Serialize};
+
+use super::track::Track;
+
+// FIXME: check direction of import.
+use super::{Merge, MergeSorted};
+
+/// The album identifier.
+#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, PartialOrd, Ord, Eq, Hash)]
+pub struct AlbumId {
+    pub year: u32,
+    pub title: String,
+}
+
+/// An album is a collection of tracks that were released together.
+#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
+pub struct Album {
+    pub id: AlbumId,
+    pub tracks: Vec<Track>,
+}
+
+impl PartialOrd for Album {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for Album {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.id.cmp(&other.id)
+    }
+}
+
+impl Merge for Album {
+    fn merge_in_place(&mut self, other: Self) {
+        assert_eq!(self.id, other.id);
+        let tracks = mem::take(&mut self.tracks);
+        self.tracks = MergeSorted::new(tracks.into_iter(), other.tracks.into_iter()).collect();
+    }
+}
diff --git a/src/collection/artist.rs b/src/collection/artist.rs
new file mode 100644
index 0000000..2d99592
--- /dev/null
+++ b/src/collection/artist.rs
@@ -0,0 +1,527 @@
+use std::{
+    fmt::{self, Debug, Display},
+    mem,
+};
+
+use paste::paste;
+use serde::{Deserialize, Serialize};
+use url::Url;
+use uuid::Uuid;
+
+use super::album::Album;
+
+// FIXME: check direction of import.
+use super::{Merge, MergeSorted};
+
+// FIXME: these imports are not acceptable
+use crate::Error;
+
+/// An object with the [`IMbid`] trait contains a [MusicBrainz
+/// Identifier](https://musicbrainz.org/doc/MusicBrainz_Identifier) (MBID).
+pub trait IMbid {
+    fn mbid(&self) -> &str;
+}
+
+/// The different URL types supported by MusicHoard.
+#[derive(Debug)]
+enum UrlType {
+    MusicBrainz,
+    MusicButler,
+    Bandcamp,
+    Qobuz,
+}
+
+/// Invalid URL error.
+// FIXME: should not be public (or at least not in this form)
+pub struct InvalidUrlError {
+    url_type: UrlType,
+    url: String,
+}
+
+impl Display for InvalidUrlError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "invalid url of type {:?}: {}", self.url_type, self.url)
+    }
+}
+
+/// MusicBrainz reference.
+#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, PartialOrd, Ord)]
+pub struct MusicBrainz(Url);
+
+impl MusicBrainz {
+    /// Validate and wrap a MusicBrainz URL.
+    pub fn new<S: AsRef<str>>(url: S) -> Result<Self, Error> {
+        let url = Url::parse(url.as_ref())?;
+
+        if !url
+            .domain()
+            .map(|u| u.ends_with("musicbrainz.org"))
+            .unwrap_or(false)
+        {
+            return Err(Self::invalid_url_error(url).into());
+        }
+
+        match url.path_segments().and_then(|mut ps| ps.nth(1)) {
+            Some(segment) => Uuid::try_parse(segment)?,
+            None => return Err(Self::invalid_url_error(url).into()),
+        };
+
+        Ok(MusicBrainz(url))
+    }
+
+    fn invalid_url_error<S: Into<String>>(url: S) -> InvalidUrlError {
+        InvalidUrlError {
+            url_type: UrlType::MusicBrainz,
+            url: url.into(),
+        }
+    }
+}
+
+impl AsRef<str> for MusicBrainz {
+    fn as_ref(&self) -> &str {
+        self.0.as_ref()
+    }
+}
+
+impl TryFrom<&str> for MusicBrainz {
+    type Error = Error;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        MusicBrainz::new(value)
+    }
+}
+
+impl Display for MusicBrainz {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+impl IMbid for MusicBrainz {
+    fn mbid(&self) -> &str {
+        // The URL is assumed to have been validated.
+        self.0.path_segments().and_then(|mut ps| ps.nth(1)).unwrap()
+    }
+}
+
+/// MusicButler reference.
+#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, PartialOrd, Ord)]
+pub struct MusicButler(Url);
+
+impl MusicButler {
+    /// Validate and wrap a MusicButler URL.
+    pub fn new<S: AsRef<str>>(url: S) -> Result<Self, Error> {
+        let url = Url::parse(url.as_ref())?;
+
+        if !url
+            .domain()
+            .map(|u| u.ends_with("musicbutler.io"))
+            .unwrap_or(false)
+        {
+            return Err(Self::invalid_url_error(url).into());
+        }
+
+        Ok(MusicButler(url))
+    }
+
+    pub fn as_str(&self) -> &str {
+        self.0.as_str()
+    }
+
+    fn invalid_url_error<S: Into<String>>(url: S) -> InvalidUrlError {
+        InvalidUrlError {
+            url_type: UrlType::MusicButler,
+            url: url.into(),
+        }
+    }
+}
+
+impl AsRef<str> for MusicButler {
+    fn as_ref(&self) -> &str {
+        self.0.as_ref()
+    }
+}
+
+impl TryFrom<&str> for MusicButler {
+    type Error = Error;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        MusicButler::new(value)
+    }
+}
+
+/// Bandcamp reference.
+#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Bandcamp(Url);
+
+impl Bandcamp {
+    /// Validate and wrap a Bandcamp URL.
+    pub fn new<S: AsRef<str>>(url: S) -> Result<Self, Error> {
+        let url = Url::parse(url.as_ref())?;
+
+        if !url
+            .domain()
+            .map(|u| u.ends_with("bandcamp.com"))
+            .unwrap_or(false)
+        {
+            return Err(Self::invalid_url_error(url).into());
+        }
+
+        Ok(Bandcamp(url))
+    }
+
+    pub fn as_str(&self) -> &str {
+        self.0.as_str()
+    }
+
+    fn invalid_url_error<S: Into<String>>(url: S) -> InvalidUrlError {
+        InvalidUrlError {
+            url_type: UrlType::Bandcamp,
+            url: url.into(),
+        }
+    }
+}
+
+impl AsRef<str> for Bandcamp {
+    fn as_ref(&self) -> &str {
+        self.0.as_ref()
+    }
+}
+
+impl TryFrom<&str> for Bandcamp {
+    type Error = Error;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        Bandcamp::new(value)
+    }
+}
+
+/// Qobuz reference.
+#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Qobuz(Url);
+
+impl Qobuz {
+    /// Validate and wrap a Qobuz URL.
+    pub fn new<S: AsRef<str>>(url: S) -> Result<Self, Error> {
+        let url = Url::parse(url.as_ref())?;
+
+        if !url
+            .domain()
+            .map(|u| u.ends_with("qobuz.com"))
+            .unwrap_or(false)
+        {
+            return Err(Self::invalid_url_error(url).into());
+        }
+
+        Ok(Qobuz(url))
+    }
+
+    fn invalid_url_error<S: Into<String>>(url: S) -> InvalidUrlError {
+        InvalidUrlError {
+            url_type: UrlType::Qobuz,
+            url: url.into(),
+        }
+    }
+}
+
+impl AsRef<str> for Qobuz {
+    fn as_ref(&self) -> &str {
+        self.0.as_ref()
+    }
+}
+
+impl TryFrom<&str> for Qobuz {
+    type Error = Error;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        Qobuz::new(value)
+    }
+}
+
+impl Display for Qobuz {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+/// The artist identifier.
+#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ArtistId {
+    pub name: String,
+}
+
+impl AsRef<ArtistId> for ArtistId {
+    fn as_ref(&self) -> &ArtistId {
+        self
+    }
+}
+
+impl ArtistId {
+    pub fn new<S: Into<String>>(name: S) -> ArtistId {
+        ArtistId { name: name.into() }
+    }
+}
+
+impl Display for ArtistId {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.name)
+    }
+}
+
+/// The artist properties.
+#[derive(Clone, Debug, Default, Deserialize, Serialize, PartialEq, Eq)]
+pub struct ArtistProperties {
+    pub musicbrainz: Option<MusicBrainz>,
+    pub musicbutler: Vec<MusicButler>,
+    pub bandcamp: Vec<Bandcamp>,
+    pub qobuz: Option<Qobuz>,
+}
+
+impl Merge for ArtistProperties {
+    fn merge_in_place(&mut self, other: Self) {
+        self.musicbrainz = self.musicbrainz.take().or(other.musicbrainz);
+        Self::merge_vecs(&mut self.musicbutler, other.musicbutler);
+        Self::merge_vecs(&mut self.bandcamp, other.bandcamp);
+        self.qobuz = self.qobuz.take().or(other.qobuz);
+    }
+}
+
+/// An artist.
+#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
+pub struct Artist {
+    pub id: ArtistId,
+    pub sort: Option<ArtistId>,
+    pub properties: ArtistProperties,
+    pub albums: Vec<Album>,
+}
+
+macro_rules! artist_unique_url_dispatch {
+    ($field:ident) => {
+        paste! {
+            pub fn [<add_ $field _url>]<S: AsRef<str>>(&mut self, url: S) -> Result<(), Error> {
+                Self::add_unique_url(&mut self.properties.$field, url)
+            }
+
+            pub fn [<remove_ $field _url>]<S: AsRef<str>>(&mut self, url: S) -> Result<(), Error> {
+                Self::remove_unique_url(&mut self.properties.$field, url)
+            }
+
+            pub fn [<set_ $field _url>]<S: AsRef<str>>(&mut self, url: S) -> Result<(), Error> {
+                Self::set_unique_url(&mut self.properties.$field, url)
+            }
+
+            pub fn [<clear_ $field _url>](&mut self) {
+                Self::clear_unique_url(&mut self.properties.$field);
+            }
+        }
+    };
+}
+
+macro_rules! artist_multi_url_dispatch {
+    ($field:ident) => {
+        paste! {
+            pub fn [<add_ $field _urls>]<S: AsRef<str>>(&mut self, urls: Vec<S>) -> Result<(), Error> {
+                Self::add_multi_urls(&mut self.properties.$field, urls)
+            }
+
+            pub fn [<remove_ $field _urls>]<S: AsRef<str>>(&mut self, urls: Vec<S>) -> Result<(), Error> {
+                Self::remove_multi_urls(&mut self.properties.$field, urls)
+            }
+
+            pub fn [<set_ $field _urls>]<S: AsRef<str>>(&mut self, urls: Vec<S>) -> Result<(), Error> {
+                Self::set_multi_urls(&mut self.properties.$field, urls)
+            }
+
+            pub fn [<clear_ $field _urls>](&mut self) {
+                Self::clear_multi_urls(&mut self.properties.$field);
+            }
+        }
+    };
+}
+
+impl Artist {
+    /// Create new [`Artist`] with the given [`ArtistId`].
+    pub fn new<ID: Into<ArtistId>>(id: ID) -> Self {
+        Artist {
+            id: id.into(),
+            sort: None,
+            properties: ArtistProperties::default(),
+            albums: vec![],
+        }
+    }
+
+    fn get_sort_key(&self) -> &ArtistId {
+        self.sort.as_ref().unwrap_or(&self.id)
+    }
+
+    pub fn set_sort_key<SORT: Into<ArtistId>>(&mut self, sort: SORT) {
+        self.sort = Some(sort.into());
+    }
+
+    pub fn clear_sort_key(&mut self) {
+        _ = self.sort.take();
+    }
+
+    fn add_unique_url<S: AsRef<str>, T: for<'a> TryFrom<&'a str, Error = Error> + Eq + Display>(
+        container: &mut Option<T>,
+        url: S,
+    ) -> Result<(), Error> {
+        let url: T = url.as_ref().try_into()?;
+
+        match container {
+            Some(current) => {
+                if current != &url {
+                    return Err(Error::CollectionError(format!(
+                        "artist already has a different URL: {}",
+                        current
+                    )));
+                }
+            }
+            None => {
+                _ = container.insert(url);
+            }
+        }
+
+        Ok(())
+    }
+
+    fn remove_unique_url<S: AsRef<str>, T: for<'a> TryFrom<&'a str, Error = Error> + Eq>(
+        container: &mut Option<T>,
+        url: S,
+    ) -> Result<(), Error> {
+        let url: T = url.as_ref().try_into()?;
+
+        if container == &Some(url) {
+            _ = container.take();
+        }
+
+        Ok(())
+    }
+
+    fn set_unique_url<S: AsRef<str>, T: for<'a> TryFrom<&'a str, Error = Error>>(
+        container: &mut Option<T>,
+        url: S,
+    ) -> Result<(), Error> {
+        _ = container.insert(url.as_ref().try_into()?);
+        Ok(())
+    }
+
+    fn clear_unique_url<T>(container: &mut Option<T>) {
+        _ = container.take();
+    }
+
+    fn add_multi_urls<S: AsRef<str>, T: for<'a> TryFrom<&'a str, Error = Error> + Eq>(
+        container: &mut Vec<T>,
+        urls: Vec<S>,
+    ) -> Result<(), Error> {
+        let mut new_urls = urls
+            .iter()
+            .map(|url| url.as_ref().try_into())
+            .filter(|res| {
+                res.as_ref()
+                    .map(|url| !container.contains(url))
+                    .unwrap_or(true) // Propagate errors.
+            })
+            .collect::<Result<Vec<T>, Error>>()?;
+
+        container.append(&mut new_urls);
+        Ok(())
+    }
+
+    fn remove_multi_urls<S: AsRef<str>, T: for<'a> TryFrom<&'a str, Error = Error> + Eq>(
+        container: &mut Vec<T>,
+        urls: Vec<S>,
+    ) -> Result<(), Error> {
+        let urls = urls
+            .iter()
+            .map(|url| url.as_ref().try_into())
+            .collect::<Result<Vec<T>, Error>>()?;
+
+        container.retain(|url| !urls.contains(url));
+        Ok(())
+    }
+
+    fn set_multi_urls<S: AsRef<str>, T: for<'a> TryFrom<&'a str, Error = Error>>(
+        container: &mut Vec<T>,
+        urls: Vec<S>,
+    ) -> Result<(), Error> {
+        let mut urls = urls
+            .iter()
+            .map(|url| url.as_ref().try_into())
+            .collect::<Result<Vec<T>, Error>>()?;
+
+        container.clear();
+        container.append(&mut urls);
+        Ok(())
+    }
+
+    fn clear_multi_urls<T>(container: &mut Vec<T>) {
+        container.clear();
+    }
+
+    artist_unique_url_dispatch!(musicbrainz);
+
+    artist_multi_url_dispatch!(musicbutler);
+
+    artist_multi_url_dispatch!(bandcamp);
+
+    artist_unique_url_dispatch!(qobuz);
+}
+
+impl PartialOrd for Artist {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for Artist {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.get_sort_key().cmp(other.get_sort_key())
+    }
+}
+
+impl Merge for Artist {
+    fn merge_in_place(&mut self, other: Self) {
+        assert_eq!(self.id, other.id);
+        self.sort = self.sort.take().or(other.sort);
+        self.properties.merge_in_place(other.properties);
+        let albums = mem::take(&mut self.albums);
+        self.albums = MergeSorted::new(albums.into_iter(), other.albums.into_iter()).collect();
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn musicbrainz() {
+        let uuid = "d368baa8-21ca-4759-9731-0b2753071ad8";
+        let url = format!("https://musicbrainz.org/artist/{uuid}");
+        let mb = MusicBrainz::new(&url).unwrap();
+        assert_eq!(url, mb.as_ref());
+        assert_eq!(uuid, mb.mbid());
+
+        let url = "not a url at all".to_string();
+        let expected_error: Error = url::ParseError::RelativeUrlWithoutBase.into();
+        let actual_error = MusicBrainz::new(url).unwrap_err();
+        assert_eq!(actual_error, expected_error);
+        assert_eq!(actual_error.to_string(), expected_error.to_string());
+
+        let url = "https://musicbrainz.org/artist/i-am-not-a-uuid".to_string();
+        let expected_error: Error = Uuid::try_parse("i-am-not-a-uuid").unwrap_err().into();
+        let actual_error = MusicBrainz::new(url).unwrap_err();
+        assert_eq!(actual_error, expected_error);
+        assert_eq!(actual_error.to_string(), expected_error.to_string());
+
+        let url = "https://musicbrainz.org/artist".to_string();
+        let expected_error: Error = InvalidUrlError {
+            url_type: UrlType::MusicBrainz,
+            url: url.clone(),
+        }
+        .into();
+        let actual_error = MusicBrainz::new(&url).unwrap_err();
+        assert_eq!(actual_error, expected_error);
+        assert_eq!(actual_error.to_string(), expected_error.to_string());
+    }
+}
diff --git a/src/collection/mod.rs b/src/collection/mod.rs
index 25e6876..b10883d 100644
--- a/src/collection/mod.rs
+++ b/src/collection/mod.rs
@@ -1,572 +1,8 @@
-use std::{
-    cmp::Ordering,
-    fmt::{self, Debug, Display},
-    iter::Peekable,
-    mem,
-};
+pub mod album;
+pub mod artist;
+pub mod track;
 
-use paste::paste;
-use serde::{Deserialize, Serialize};
-use url::Url;
-use uuid::Uuid;
-
-// FIXME: these imports are not acceptable
-use crate::Error;
-
-/// An object with the [`IMbid`] trait contains a [MusicBrainz
-/// Identifier](https://musicbrainz.org/doc/MusicBrainz_Identifier) (MBID).
-pub trait IMbid {
-    fn mbid(&self) -> &str;
-}
-
-/// The different URL types supported by MusicHoard.
-#[derive(Debug)]
-enum UrlType {
-    MusicBrainz,
-    MusicButler,
-    Bandcamp,
-    Qobuz,
-}
-
-/// Invalid URL error.
-// FIXME: should not be public (or at least not in this form)
-pub struct InvalidUrlError {
-    url_type: UrlType,
-    url: String,
-}
-
-impl Display for InvalidUrlError {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "invalid url of type {:?}: {}", self.url_type, self.url)
-    }
-}
-
-/// MusicBrainz reference.
-#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, PartialOrd, Ord)]
-pub struct MusicBrainz(Url);
-
-impl MusicBrainz {
-    /// Validate and wrap a MusicBrainz URL.
-    pub fn new<S: AsRef<str>>(url: S) -> Result<Self, Error> {
-        let url = Url::parse(url.as_ref())?;
-
-        if !url
-            .domain()
-            .map(|u| u.ends_with("musicbrainz.org"))
-            .unwrap_or(false)
-        {
-            return Err(Self::invalid_url_error(url).into());
-        }
-
-        match url.path_segments().and_then(|mut ps| ps.nth(1)) {
-            Some(segment) => Uuid::try_parse(segment)?,
-            None => return Err(Self::invalid_url_error(url).into()),
-        };
-
-        Ok(MusicBrainz(url))
-    }
-
-    fn invalid_url_error<S: Into<String>>(url: S) -> InvalidUrlError {
-        InvalidUrlError {
-            url_type: UrlType::MusicBrainz,
-            url: url.into(),
-        }
-    }
-}
-
-impl AsRef<str> for MusicBrainz {
-    fn as_ref(&self) -> &str {
-        self.0.as_ref()
-    }
-}
-
-impl TryFrom<&str> for MusicBrainz {
-    type Error = Error;
-
-    fn try_from(value: &str) -> Result<Self, Self::Error> {
-        MusicBrainz::new(value)
-    }
-}
-
-impl Display for MusicBrainz {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{}", self.0)
-    }
-}
-
-impl IMbid for MusicBrainz {
-    fn mbid(&self) -> &str {
-        // The URL is assumed to have been validated.
-        self.0.path_segments().and_then(|mut ps| ps.nth(1)).unwrap()
-    }
-}
-
-/// MusicButler reference.
-#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, PartialOrd, Ord)]
-pub struct MusicButler(Url);
-
-impl MusicButler {
-    /// Validate and wrap a MusicButler URL.
-    pub fn new<S: AsRef<str>>(url: S) -> Result<Self, Error> {
-        let url = Url::parse(url.as_ref())?;
-
-        if !url
-            .domain()
-            .map(|u| u.ends_with("musicbutler.io"))
-            .unwrap_or(false)
-        {
-            return Err(Self::invalid_url_error(url).into());
-        }
-
-        Ok(MusicButler(url))
-    }
-
-    pub fn as_str(&self) -> &str {
-        self.0.as_str()
-    }
-
-    fn invalid_url_error<S: Into<String>>(url: S) -> InvalidUrlError {
-        InvalidUrlError {
-            url_type: UrlType::MusicButler,
-            url: url.into(),
-        }
-    }
-}
-
-impl AsRef<str> for MusicButler {
-    fn as_ref(&self) -> &str {
-        self.0.as_ref()
-    }
-}
-
-impl TryFrom<&str> for MusicButler {
-    type Error = Error;
-
-    fn try_from(value: &str) -> Result<Self, Self::Error> {
-        MusicButler::new(value)
-    }
-}
-
-/// Bandcamp reference.
-#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, PartialOrd, Ord)]
-pub struct Bandcamp(Url);
-
-impl Bandcamp {
-    /// Validate and wrap a Bandcamp URL.
-    pub fn new<S: AsRef<str>>(url: S) -> Result<Self, Error> {
-        let url = Url::parse(url.as_ref())?;
-
-        if !url
-            .domain()
-            .map(|u| u.ends_with("bandcamp.com"))
-            .unwrap_or(false)
-        {
-            return Err(Self::invalid_url_error(url).into());
-        }
-
-        Ok(Bandcamp(url))
-    }
-
-    pub fn as_str(&self) -> &str {
-        self.0.as_str()
-    }
-
-    fn invalid_url_error<S: Into<String>>(url: S) -> InvalidUrlError {
-        InvalidUrlError {
-            url_type: UrlType::Bandcamp,
-            url: url.into(),
-        }
-    }
-}
-
-impl AsRef<str> for Bandcamp {
-    fn as_ref(&self) -> &str {
-        self.0.as_ref()
-    }
-}
-
-impl TryFrom<&str> for Bandcamp {
-    type Error = Error;
-
-    fn try_from(value: &str) -> Result<Self, Self::Error> {
-        Bandcamp::new(value)
-    }
-}
-
-/// Qobuz reference.
-#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, PartialOrd, Ord)]
-pub struct Qobuz(Url);
-
-impl Qobuz {
-    /// Validate and wrap a Qobuz URL.
-    pub fn new<S: AsRef<str>>(url: S) -> Result<Self, Error> {
-        let url = Url::parse(url.as_ref())?;
-
-        if !url
-            .domain()
-            .map(|u| u.ends_with("qobuz.com"))
-            .unwrap_or(false)
-        {
-            return Err(Self::invalid_url_error(url).into());
-        }
-
-        Ok(Qobuz(url))
-    }
-
-    fn invalid_url_error<S: Into<String>>(url: S) -> InvalidUrlError {
-        InvalidUrlError {
-            url_type: UrlType::Qobuz,
-            url: url.into(),
-        }
-    }
-}
-
-impl AsRef<str> for Qobuz {
-    fn as_ref(&self) -> &str {
-        self.0.as_ref()
-    }
-}
-
-impl TryFrom<&str> for Qobuz {
-    type Error = Error;
-
-    fn try_from(value: &str) -> Result<Self, Self::Error> {
-        Qobuz::new(value)
-    }
-}
-
-impl Display for Qobuz {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{}", self.0)
-    }
-}
-
-/// The track file format.
-#[derive(Clone, Copy, Debug, Deserialize, Serialize, PartialEq, Eq, Hash)]
-pub enum Format {
-    Flac,
-    Mp3,
-}
-
-/// The track quality. Combines format and bitrate information.
-#[derive(Clone, Copy, Debug, Deserialize, Serialize, PartialEq, Eq)]
-pub struct Quality {
-    pub format: Format,
-    pub bitrate: u32,
-}
-
-/// The track identifier.
-#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub struct TrackId {
-    pub number: u32,
-    pub title: String,
-}
-
-/// A single track on an album.
-#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
-pub struct Track {
-    pub id: TrackId,
-    pub artist: Vec<String>,
-    pub quality: Quality,
-}
-
-impl PartialOrd for Track {
-    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
-        Some(self.cmp(other))
-    }
-}
-
-impl Ord for Track {
-    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
-        self.id.cmp(&other.id)
-    }
-}
-
-impl Merge for Track {
-    fn merge_in_place(&mut self, other: Self) {
-        assert_eq!(self.id, other.id);
-    }
-}
-
-/// The album identifier.
-#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, PartialOrd, Ord, Eq, Hash)]
-pub struct AlbumId {
-    pub year: u32,
-    pub title: String,
-}
-
-/// An album is a collection of tracks that were released together.
-#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
-pub struct Album {
-    pub id: AlbumId,
-    pub tracks: Vec<Track>,
-}
-
-impl PartialOrd for Album {
-    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
-        Some(self.cmp(other))
-    }
-}
-
-impl Ord for Album {
-    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
-        self.id.cmp(&other.id)
-    }
-}
-
-impl Merge for Album {
-    fn merge_in_place(&mut self, other: Self) {
-        assert_eq!(self.id, other.id);
-        let tracks = mem::take(&mut self.tracks);
-        self.tracks = MergeSorted::new(tracks.into_iter(), other.tracks.into_iter()).collect();
-    }
-}
-
-/// The artist identifier.
-#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub struct ArtistId {
-    pub name: String,
-}
-
-impl AsRef<ArtistId> for ArtistId {
-    fn as_ref(&self) -> &ArtistId {
-        self
-    }
-}
-
-impl ArtistId {
-    pub fn new<S: Into<String>>(name: S) -> ArtistId {
-        ArtistId { name: name.into() }
-    }
-}
-
-impl Display for ArtistId {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{}", self.name)
-    }
-}
-
-/// The artist properties.
-#[derive(Clone, Debug, Default, Deserialize, Serialize, PartialEq, Eq)]
-pub struct ArtistProperties {
-    pub musicbrainz: Option<MusicBrainz>,
-    pub musicbutler: Vec<MusicButler>,
-    pub bandcamp: Vec<Bandcamp>,
-    pub qobuz: Option<Qobuz>,
-}
-
-impl Merge for ArtistProperties {
-    fn merge_in_place(&mut self, other: Self) {
-        self.musicbrainz = self.musicbrainz.take().or(other.musicbrainz);
-        Self::merge_vecs(&mut self.musicbutler, other.musicbutler);
-        Self::merge_vecs(&mut self.bandcamp, other.bandcamp);
-        self.qobuz = self.qobuz.take().or(other.qobuz);
-    }
-}
-
-/// An artist.
-#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
-pub struct Artist {
-    pub id: ArtistId,
-    pub sort: Option<ArtistId>,
-    pub properties: ArtistProperties,
-    pub albums: Vec<Album>,
-}
-
-macro_rules! artist_unique_url_dispatch {
-    ($field:ident) => {
-        paste! {
-            pub fn [<add_ $field _url>]<S: AsRef<str>>(&mut self, url: S) -> Result<(), Error> {
-                Self::add_unique_url(&mut self.properties.$field, url)
-            }
-
-            pub fn [<remove_ $field _url>]<S: AsRef<str>>(&mut self, url: S) -> Result<(), Error> {
-                Self::remove_unique_url(&mut self.properties.$field, url)
-            }
-
-            pub fn [<set_ $field _url>]<S: AsRef<str>>(&mut self, url: S) -> Result<(), Error> {
-                Self::set_unique_url(&mut self.properties.$field, url)
-            }
-
-            pub fn [<clear_ $field _url>](&mut self) {
-                Self::clear_unique_url(&mut self.properties.$field);
-            }
-        }
-    };
-}
-
-macro_rules! artist_multi_url_dispatch {
-    ($field:ident) => {
-        paste! {
-            pub fn [<add_ $field _urls>]<S: AsRef<str>>(&mut self, urls: Vec<S>) -> Result<(), Error> {
-                Self::add_multi_urls(&mut self.properties.$field, urls)
-            }
-
-            pub fn [<remove_ $field _urls>]<S: AsRef<str>>(&mut self, urls: Vec<S>) -> Result<(), Error> {
-                Self::remove_multi_urls(&mut self.properties.$field, urls)
-            }
-
-            pub fn [<set_ $field _urls>]<S: AsRef<str>>(&mut self, urls: Vec<S>) -> Result<(), Error> {
-                Self::set_multi_urls(&mut self.properties.$field, urls)
-            }
-
-            pub fn [<clear_ $field _urls>](&mut self) {
-                Self::clear_multi_urls(&mut self.properties.$field);
-            }
-        }
-    };
-}
-
-impl Artist {
-    /// Create new [`Artist`] with the given [`ArtistId`].
-    pub fn new<ID: Into<ArtistId>>(id: ID) -> Self {
-        Artist {
-            id: id.into(),
-            sort: None,
-            properties: ArtistProperties::default(),
-            albums: vec![],
-        }
-    }
-
-    fn get_sort_key(&self) -> &ArtistId {
-        self.sort.as_ref().unwrap_or(&self.id)
-    }
-
-    pub fn set_sort_key<SORT: Into<ArtistId>>(&mut self, sort: SORT) {
-        self.sort = Some(sort.into());
-    }
-
-    pub fn clear_sort_key(&mut self) {
-        _ = self.sort.take();
-    }
-
-    fn add_unique_url<S: AsRef<str>, T: for<'a> TryFrom<&'a str, Error = Error> + Eq + Display>(
-        container: &mut Option<T>,
-        url: S,
-    ) -> Result<(), Error> {
-        let url: T = url.as_ref().try_into()?;
-
-        match container {
-            Some(current) => {
-                if current != &url {
-                    return Err(Error::CollectionError(format!(
-                        "artist already has a different URL: {}",
-                        current
-                    )));
-                }
-            }
-            None => {
-                _ = container.insert(url);
-            }
-        }
-
-        Ok(())
-    }
-
-    fn remove_unique_url<S: AsRef<str>, T: for<'a> TryFrom<&'a str, Error = Error> + Eq>(
-        container: &mut Option<T>,
-        url: S,
-    ) -> Result<(), Error> {
-        let url: T = url.as_ref().try_into()?;
-
-        if container == &Some(url) {
-            _ = container.take();
-        }
-
-        Ok(())
-    }
-
-    fn set_unique_url<S: AsRef<str>, T: for<'a> TryFrom<&'a str, Error = Error>>(
-        container: &mut Option<T>,
-        url: S,
-    ) -> Result<(), Error> {
-        _ = container.insert(url.as_ref().try_into()?);
-        Ok(())
-    }
-
-    fn clear_unique_url<T>(container: &mut Option<T>) {
-        _ = container.take();
-    }
-
-    fn add_multi_urls<S: AsRef<str>, T: for<'a> TryFrom<&'a str, Error = Error> + Eq>(
-        container: &mut Vec<T>,
-        urls: Vec<S>,
-    ) -> Result<(), Error> {
-        let mut new_urls = urls
-            .iter()
-            .map(|url| url.as_ref().try_into())
-            .filter(|res| {
-                res.as_ref()
-                    .map(|url| !container.contains(url))
-                    .unwrap_or(true) // Propagate errors.
-            })
-            .collect::<Result<Vec<T>, Error>>()?;
-
-        container.append(&mut new_urls);
-        Ok(())
-    }
-
-    fn remove_multi_urls<S: AsRef<str>, T: for<'a> TryFrom<&'a str, Error = Error> + Eq>(
-        container: &mut Vec<T>,
-        urls: Vec<S>,
-    ) -> Result<(), Error> {
-        let urls = urls
-            .iter()
-            .map(|url| url.as_ref().try_into())
-            .collect::<Result<Vec<T>, Error>>()?;
-
-        container.retain(|url| !urls.contains(url));
-        Ok(())
-    }
-
-    fn set_multi_urls<S: AsRef<str>, T: for<'a> TryFrom<&'a str, Error = Error>>(
-        container: &mut Vec<T>,
-        urls: Vec<S>,
-    ) -> Result<(), Error> {
-        let mut urls = urls
-            .iter()
-            .map(|url| url.as_ref().try_into())
-            .collect::<Result<Vec<T>, Error>>()?;
-
-        container.clear();
-        container.append(&mut urls);
-        Ok(())
-    }
-
-    fn clear_multi_urls<T>(container: &mut Vec<T>) {
-        container.clear();
-    }
-
-    artist_unique_url_dispatch!(musicbrainz);
-
-    artist_multi_url_dispatch!(musicbutler);
-
-    artist_multi_url_dispatch!(bandcamp);
-
-    artist_unique_url_dispatch!(qobuz);
-}
-
-impl PartialOrd for Artist {
-    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
-        Some(self.cmp(other))
-    }
-}
-
-impl Ord for Artist {
-    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
-        self.get_sort_key().cmp(other.get_sort_key())
-    }
-}
-
-impl Merge for Artist {
-    fn merge_in_place(&mut self, other: Self) {
-        assert_eq!(self.id, other.id);
-        self.sort = self.sort.take().or(other.sort);
-        self.properties.merge_in_place(other.properties);
-        let albums = mem::take(&mut self.albums);
-        self.albums = MergeSorted::new(albums.into_iter(), other.albums.into_iter()).collect();
-    }
-}
+use std::{cmp::Ordering, iter::Peekable};
 
 // FIXME: should not be public
 pub trait Merge {
@@ -633,41 +69,5 @@ where
     }
 }
 
-/// The collection type. Currently, a collection is a list of artists.
-pub type Collection = Vec<Artist>;
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn musicbrainz() {
-        let uuid = "d368baa8-21ca-4759-9731-0b2753071ad8";
-        let url = format!("https://musicbrainz.org/artist/{uuid}");
-        let mb = MusicBrainz::new(&url).unwrap();
-        assert_eq!(url, mb.as_ref());
-        assert_eq!(uuid, mb.mbid());
-
-        let url = "not a url at all".to_string();
-        let expected_error: Error = url::ParseError::RelativeUrlWithoutBase.into();
-        let actual_error = MusicBrainz::new(url).unwrap_err();
-        assert_eq!(actual_error, expected_error);
-        assert_eq!(actual_error.to_string(), expected_error.to_string());
-
-        let url = "https://musicbrainz.org/artist/i-am-not-a-uuid".to_string();
-        let expected_error: Error = Uuid::try_parse("i-am-not-a-uuid").unwrap_err().into();
-        let actual_error = MusicBrainz::new(url).unwrap_err();
-        assert_eq!(actual_error, expected_error);
-        assert_eq!(actual_error.to_string(), expected_error.to_string());
-
-        let url = "https://musicbrainz.org/artist".to_string();
-        let expected_error: Error = InvalidUrlError {
-            url_type: UrlType::MusicBrainz,
-            url: url.clone(),
-        }
-        .into();
-        let actual_error = MusicBrainz::new(&url).unwrap_err();
-        assert_eq!(actual_error, expected_error);
-        assert_eq!(actual_error.to_string(), expected_error.to_string());
-    }
-}
+/// The collection alias type for convenience.
+pub type Collection = Vec<artist::Artist>;
diff --git a/src/collection/track.rs b/src/collection/track.rs
new file mode 100644
index 0000000..f4279fd
--- /dev/null
+++ b/src/collection/track.rs
@@ -0,0 +1,51 @@
+use serde::{Deserialize, Serialize};
+
+// FIXME: check direction of import.
+use super::Merge;
+
+/// The track file format.
+#[derive(Clone, Copy, Debug, Deserialize, Serialize, PartialEq, Eq, Hash)]
+pub enum Format {
+    Flac,
+    Mp3,
+}
+
+/// The track quality. Combines format and bitrate information.
+#[derive(Clone, Copy, Debug, Deserialize, Serialize, PartialEq, Eq)]
+pub struct Quality {
+    pub format: Format,
+    pub bitrate: u32,
+}
+
+/// The track identifier.
+#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct TrackId {
+    pub number: u32,
+    pub title: String,
+}
+
+/// A single track on an album.
+#[derive(Clone, Debug, Deserialize, Serialize, PartialEq, Eq)]
+pub struct Track {
+    pub id: TrackId,
+    pub artist: Vec<String>,
+    pub quality: Quality,
+}
+
+impl PartialOrd for Track {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for Track {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.id.cmp(&other.id)
+    }
+}
+
+impl Merge for Track {
+    fn merge_in_place(&mut self, other: Self) {
+        assert_eq!(self.id, other.id);
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index aeee122..aaddaea 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -10,15 +10,17 @@ use std::{
     mem,
 };
 
-use collection::{InvalidUrlError, Merge};
+use collection::{artist::InvalidUrlError, Merge};
 use database::IDatabase;
 use library::{ILibrary, Item, Query};
 use paste::paste;
 
-// TODO: validate the re-exports.
+// FIXME: validate the re-exports.
 pub use collection::{
-    Album, AlbumId, Artist, ArtistId, ArtistProperties, Bandcamp, Collection, Format, MusicBrainz,
-    MusicButler, Qobuz, Quality, Track, TrackId,
+    album::{Album, AlbumId},
+    artist::{Artist, ArtistId, ArtistProperties, Bandcamp, MusicBrainz, MusicButler, Qobuz},
+    track::{Format, Quality, Track, TrackId},
+    Collection,
 };
 
 /// Error type for `musichoard`.
diff --git a/src/library/beets/mod.rs b/src/library/beets/mod.rs
index ffddae5..0c6cee1 100644
--- a/src/library/beets/mod.rs
+++ b/src/library/beets/mod.rs
@@ -4,7 +4,7 @@
 #[cfg(test)]
 use mockall::automock;
 
-use crate::collection::Format;
+use crate::collection::track::Format;
 
 use super::{Error, Field, ILibrary, Item, Query};
 
diff --git a/src/library/mod.rs b/src/library/mod.rs
index 3d2ae87..645081a 100644
--- a/src/library/mod.rs
+++ b/src/library/mod.rs
@@ -5,7 +5,7 @@ use std::{collections::HashSet, fmt, num::ParseIntError, str::Utf8Error};
 #[cfg(test)]
 use mockall::automock;
 
-use crate::collection::Format;
+use crate::collection::track::Format;
 
 #[cfg(feature = "library-beets")]
 pub mod beets;
diff --git a/src/library/testmod.rs b/src/library/testmod.rs
index 284ec54..638aeb2 100644
--- a/src/library/testmod.rs
+++ b/src/library/testmod.rs
@@ -1,6 +1,6 @@
 use once_cell::sync::Lazy;
 
-use crate::{collection::Format, library::Item};
+use crate::{collection::track::Format, library::Item};
 
 pub static LIBRARY_ITEMS: Lazy<Vec<Item>> = Lazy::new(|| -> Vec<Item> {
     vec![
diff --git a/src/testlib.rs b/src/testlib.rs
index 4015bef..519757d 100644
--- a/src/testlib.rs
+++ b/src/testlib.rs
@@ -1,6 +1,7 @@
 use crate::collection::{
-    Album, AlbumId, Artist, ArtistId, ArtistProperties, Bandcamp, Format, MusicBrainz, MusicButler,
-    Qobuz, Quality, Track, TrackId,
+    album::{Album, AlbumId},
+    artist::{Artist, ArtistId, ArtistProperties, Bandcamp, MusicBrainz, MusicButler, Qobuz},
+    track::{Format, Quality, Track, TrackId},
 };
 use once_cell::sync::Lazy;
 
diff --git a/src/tests/mod.rs b/src/tests/mod.rs
index 14e31b0..93589e3 100644
--- a/src/tests/mod.rs
+++ b/src/tests/mod.rs
@@ -1,7 +1,11 @@
 use mockall::predicate;
 
 use super::*;
-use collection::{Bandcamp, Format, Merge, MusicBrainz, MusicButler, Qobuz};
+use collection::{
+    artist::{Bandcamp, MusicBrainz, MusicButler, Qobuz},
+    track::Format,
+    Merge,
+};
 use database::MockIDatabase;
 use library::{testmod::LIBRARY_ITEMS, MockILibrary};
 use testlib::{FULL_COLLECTION, LIBRARY_COLLECTION};