Include has poor performance on large arrays

lib/mongoid/identity_map.rb

@@ -4,6 +4,21 @@ module Mongoid #:nodoc:
   # Defines behaviour for the identity map in Mongoid.
   class IdentityMap < Hash
 
+    # Clear the many documents.
+    #
+    # @example Clear the docs.
+    #   identity_map.clear_many(Post, selector)
+    #
+    # @param [ Class ] klass The klass to clear.
+    # @param [ Hash ] selector The selector to identify it.
+    #
+    # @return [ Array<Document> ] The documents.
+    #
+    # @since 2.4.10
+    def clear_many(klass, selector)
+      (documents_for(klass)[selector] ||= []).clear
+    end
+
     # Get a document from the identity map by its id.
     #
     # @example Get the document from the map.
@@ -62,8 +77,7 @@ def set(document)
     #
     # @since 2.2.0
     def set_many(document, selector)
-      documents = documents_for(document.class)[selector] ||= []
-      documents.push(document) unless documents.include?(document)
+      (documents_for(document.class)[selector] ||= []).push(document)
     end
 
     # Set a document in the identity map for the provided selector.

lib/mongoid/relations/referenced/many.rb

@@ -563,9 +563,15 @@ def criteria(metadata, object, type = nil)
           #
           # @since 2.2.0
           def eager_load(metadata, ids)
+            cleared = false
             klass, foreign_key = metadata.klass, metadata.foreign_key
             klass.any_in(foreign_key => ids).each do |doc|
-              IdentityMap.set_many(doc, foreign_key => doc.send(foreign_key))
+              base_id = doc.send(foreign_key)
+              unless cleared
+                IdentityMap.clear_many(klass, foreign_key => base_id)
+                cleared = true
+              end
+              IdentityMap.set_many(doc, foreign_key => base_id)
             end
           end