Minor change for the sake of simplification and flexibility.

src/MappedCollection2.coffee

@@ -86,15 +86,30 @@ $mapInPlace = (col, iterator, ctx) ->
 
 class $MappedCollection2 extends $EventEmitter
 
+  # The dispatch function is called whenever a new item has to be
+  # processed and returns the name of the rule to use.
+  #
+  # To change the way it is dispatched, just override this it.
+  dispatch: ->
+    (@genval and (@genval.rule ? @genval.type)) ? 'unknown'
+
+  # This function is called when an item has been dispatched to a
+  # missing rule.
+  #
+  # The default behavior is to throw an error but you may instead
+  # choose to create a rule:
+  #
+  #     collection.missingRule = collection.rule
+  missingRule: (name) ->
+    throw new Error "undefined rule “#{name}”"
+
   # This option makes `set()` create missing rules when necessary
   # instead of failing.
   #
   # TODO: should be replaced by a callback for flexibility.
   createMissingRules: false
 
-  constructor: ({createMissingRules} = {}) ->
-    @createMissingRules = !!createMissingRules if createMissingRules?
-
+  constructor: ->
     # Items are stored here indexed by key.
     #
     # The prototype of this object is set to `null` to avoid pollution
@@ -117,15 +132,6 @@ class $MappedCollection2 extends $EventEmitter
     # to use the `name of @_rules` syntax.
     @_rules = Object.create null
 
-  # Register a dispatch function.
-  #
-  # The dispatch function is called whenever a new item has to be
-  # processed and returns the name of the rule to use.
-  dispatch: (fn) ->
-    return @_dispatch unless fn?
-
-    @_dispatch = fn
-
   # Register a hook to run at a given point.
   #
   # A hook receives as parameter an event object with the following
@@ -221,8 +227,8 @@ class $MappedCollection2 extends $EventEmitter
       definition.call ctx
     else
       ctx = {
-        key: definition.key
-        val: definition.val
+        key: definition?.key
+        val: definition?.val
         singleton
       }
 
@@ -270,11 +276,10 @@ class $MappedCollection2 extends $EventEmitter
   remove: (keys) ->
     @_removeItems (@_fetchItems keys)
 
-  set: (items, {add, update, remove, createMissingRules} = {}) ->
+  set: (items, {add, update, remove} = {}) ->
     add = true unless add?
     update = true unless update?
     remove = false unless remove?
-    createMissingRules = false unless createMissingRules?
 
     itemsToAdd = {}
     itemsToUpdate = {}
@@ -294,15 +299,15 @@ class $MappedCollection2 extends $EventEmitter
       return unless @_runHook 'beforeDispatch', item
 
       # Searches for a rule to handle it.
-      ruleName = @_dispatch.call item
+      ruleName = @dispatch.call item
       rule = @_rules[ruleName]
 
       unless rule?
-        @_assert(
-          @createMissingRules
-          "undefined rule “#{ruleName}”"
-        )
-        rule = @rule ruleName, {}
+        @missingRule ruleName
+
+        # The flow has not been interrupted, `missingRule()` must have
+        # created the rule.
+        rule = @_rules[ruleName]
 
       # Checks if this is a singleton.
       @_assert(
@@ -368,10 +373,6 @@ class $MappedCollection2 extends $EventEmitter
   _assert: (cond, message) ->
     throw new Error message unless cond
 
-  # Default function used for dispatching.
-  _dispatch: ->
-    (@genval and @genval.rule ? @genval.type) ? 'unknown'
-
   # Emits item related event.
   _emitEvent: (event, items) ->
     byRule = {}

src/MappedCollection2.spec.coffee

@@ -26,8 +26,8 @@ describe '$MappedCollection2', ->
 
     #------------------------------
 
-    it 'should provides genkey and genval to the dispatch function', ->
-      collection.dispatch ->
+    it 'should have genkey and genval', ->
+      collection.dispatch = ->
         $expect(@genkey).to.equal 'a key'
         $expect(@genval).to.equal 'a value'
 
@@ -39,8 +39,8 @@ describe '$MappedCollection2', ->
 
     #------------------------------
 
-    it 'should register a dispatch function', ->
-      collection.dispatch -> 'test'
+    it 'should be used to dispatch an item', ->
+      collection.dispatch = -> 'test'
 
       collection.set [
         'any value'
@@ -48,14 +48,6 @@ describe '$MappedCollection2', ->
 
       $expect(collection.getRaw('0').rule).to.equal 'test'
 
-    #------------------------------
-
-    it 'should return the dispatch function with no parameter', ->
-      dispatcher = -> 'test'
-      collection.dispatch dispatcher
-
-      $expect(collection.dispatch()).to.equal dispatcher
-
   #-------------------------------------------------------------------
 
   describe 'item hooks', ->
@@ -76,7 +68,7 @@ describe '$MappedCollection2', ->
 
         # It still is a dispatcher.
         'test'
-      collection.dispatch dispatcher
+      collection.dispatch = dispatcher
 
       beforeUpdate = $sinon.spy ->
         $expect(dispatcher.called).to.true

src/helpers.spec.coffee

@@ -16,12 +16,13 @@ describe 'Helper', ->
   collection = $set = $sum = $val = null
   beforeEach ->
     # Creates the collection.
-    collection = new $MappedCollection2 {
-      createMissingRules: true
-    }
+    collection = new $MappedCollection2()
 
     # Dispatcher used for tests.
-    collection.dispatch -> (@genkey.split '.')[0]
+    collection.dispatch = -> (@genkey.split '.')[0]
+
+    # Missing rules should be automatically created.
+    collection.missingRule = collection.rule
 
     # # Monkey patch the collection to see all emitted events.
     # emit = collection.emit