diff --git a/packages/xo-collection/src/unique-index.js b/packages/xo-collection/src/unique-index.js
new file mode 100644
index 000000000..36e6cb13f
--- /dev/null
+++ b/packages/xo-collection/src/unique-index.js
@@ -0,0 +1,123 @@
+import bind from 'lodash.bind'
+import callback from 'lodash.callback'
+
+import clearObject from './clear-object'
+import NotImplemented from './not-implemented'
+
+// ===================================================================
+
+export default class UniqueIndex {
+  constructor (computeHash) {
+    if (computeHash) {
+      this.computeHash = callback(computeHash)
+    }
+
+    this._itemByHash = Object.create(null)
+    this._keysToHash = Object.create(null)
+
+    // Bound versions of listeners.
+    this._onAdd = bind(this._onAdd, this)
+    this._onUpdate = bind(this._onUpdate, this)
+    this._onRemove = bind(this._onRemove, this)
+  }
+
+  // This method is used to compute the hash under which an item must
+  // be saved.
+  computeHash (value, key) {
+    throw new NotImplemented('this method must be overridden')
+  }
+
+  // -----------------------------------------------------------------
+
+  get items () {
+    return this._itemByHash
+  }
+
+  // -----------------------------------------------------------------
+
+  _attachCollection (collection) {
+    // Add existing entries.
+    //
+    // FIXME: I think there may be a race condition if the `add` event
+    // has not been emitted yet.
+    this._onAdd(collection.all)
+
+    collection.on('add', this._onAdd)
+    collection.on('update', this._onUpdate)
+    collection.on('remove', this._onRemove)
+  }
+
+  _detachCollection (collection) {
+    collection.removeListener('add', this._onAdd)
+    collection.removeListener('update', this._onUpdate)
+    collection.removeListener('remove', this._onRemove)
+
+    clearObject(this._itemByHash)
+    clearObject(this._keysToHash)
+  }
+
+  // -----------------------------------------------------------------
+
+  _onAdd (items) {
+    const {
+      computeHash,
+      _itemByHash: itemByHash,
+      _keysToHash: keysToHash
+    } = this
+
+    for (let key in items) {
+      const value = items[key]
+
+      const hash = computeHash(value, key)
+
+      if (hash != null) {
+        itemByHash[hash] = {key, value}
+        keysToHash[key] = hash
+      }
+    }
+  }
+
+  _onUpdate (items) {
+    const {
+      computeHash,
+      _itemByHash: itemByHash,
+      _keysToHash: keysToHash
+    } = this
+
+    for (let key in items) {
+      const value = items[key]
+
+      const prev = keysToHash[key]
+      const hash = computeHash(value, key)
+
+      // Same hash, nothing to do.
+      if (hash === prev) continue
+
+      // Removes item from the previous hash's list if any.
+      if (prev != null) delete itemByHash[prev]
+
+      // Inserts item into the new hash's list if any.
+      if (hash != null) {
+        keysToHash[key] = hash
+        itemByHash[hash] = {key, value}
+      } else {
+        delete keysToHash[key]
+      }
+    }
+  }
+
+  _onRemove (items) {
+    const {
+      _itemByHash: itemByHash,
+      _keysToHash: keysToHash
+    } = this
+
+    for (let key in items) {
+      const prev = keysToHash[key]
+      if (prev != null) {
+        delete keysToHash[key]
+        delete itemByHash[prev]
+      }
+    }
+  }
+}
diff --git a/packages/xo-collection/src/unique-index.spec.js b/packages/xo-collection/src/unique-index.spec.js
new file mode 100644
index 000000000..cc62fa4fd
--- /dev/null
+++ b/packages/xo-collection/src/unique-index.spec.js
@@ -0,0 +1,144 @@
+/* eslint-env mocha */
+
+import chai, {expect} from 'chai'
+import dirtyChai from 'dirty-chai'
+chai.use(dirtyChai)
+
+import sourceMapSupport from 'source-map-support'
+sourceMapSupport.install()
+
+import forEach from 'lodash.foreach'
+
+// -------------------------------------------------------------------
+
+import Collection from '..'
+import Index from '../unique-index'
+
+// ===================================================================
+
+const waitTicks = (n = 1) => {
+  const {nextTick} = process
+
+  return new Promise(resolve => {
+    (function waitNextTick () {
+      // The first tick is handled by Promise#then()
+      if (--n) {
+        nextTick(waitNextTick)
+      } else {
+        resolve()
+      }
+    })()
+  })
+}
+
+// ===================================================================
+
+describe('UniqueIndex', function () {
+  let col, byKey
+  const item1 = {
+    id: '2ccb8a72-dc65-48e4-88fe-45ef541f2cba',
+    key: '036dee1b-9a3b-4fb5-be8a-4f535b355581'
+  }
+  const item2 = {
+    id: '7d21dc51-4da8-4538-a2e9-dd6f4784eb76',
+    key: '103cd893-d2cc-4d37-96fd-c259ad04c0d4'
+  }
+  const item3 = {
+    id: '668c1274-4442-44a6-b99a-512188e0bb09'
+  }
+
+  beforeEach(function () {
+    col = new Collection()
+    forEach([item1, item2, item3], item => {
+      col.add(item)
+    })
+
+    byKey = new Index('key')
+
+    col.createIndex('byKey', byKey)
+
+    return waitTicks()
+  })
+
+  it('works with existing items', function () {
+    expect(col.indexes).to.eql({
+      byKey: {
+        [item1.key]: {
+          key: item1.id,
+          value: item1
+        },
+        [item2.key]: {
+          key: item2.id,
+          value: item2
+        }
+      }
+    })
+  })
+
+  it('works with added items', function () {
+    const item4 = {
+      id: '823b56c4-4b96-4f3a-9533-5d08177167ac',
+      key: '1437af14-429a-40db-8a51-8a2f5ed03201'
+    }
+
+    col.add(item4)
+
+    return waitTicks(2).then(() => {
+      expect(col.indexes).to.eql({
+        byKey: {
+          [item1.key]: {
+            key: item1.id,
+            value: item1
+          },
+          [item2.key]: {
+            key: item2.id,
+            value: item2
+          },
+          [item4.key]: {
+            key: item4.id,
+            value: item4
+          }
+        }
+      })
+    })
+  })
+
+  it('works with updated items', function () {
+    const item1bis = {
+      id: item1.id,
+      key: 'e03d4a3a-0331-4aca-97a2-016bbd43a29b'
+    }
+
+    col.update(item1bis)
+
+    return waitTicks(2).then(() => {
+      expect(col.indexes).to.eql({
+        byKey: {
+          [item1bis.key]: {
+            key: item1.id,
+            value: item1bis
+          },
+          [item2.key]: {
+            key: item2.id,
+            value: item2
+          }
+        }
+      })
+    })
+  })
+
+  it('works with removed items', function () {
+    col.remove(item2)
+
+    return waitTicks(2).then(() => {
+      expect(col.indexes).to.eql({
+        byKey: {
+          [item1.key]: {
+            key: item1.id,
+            value: item1
+          }
+        }
+      })
+    })
+  })
+})
diff --git a/packages/xo-collection/unique-index.js b/packages/xo-collection/unique-index.js
new file mode 100644
index 000000000..9f9d92cd5
--- /dev/null
+++ b/packages/xo-collection/unique-index.js
@@ -0,0 +1 @@
+module.exports = require('./dist/unique-index')