1 files changed, 77 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/relation/spawn_methods.rb b/activerecord/lib/active_record/relation/spawn_methods.rb
new file mode 100644
index 0000000000..7874c4c35a
--- /dev/null
+++ b/activerecord/lib/active_record/relation/spawn_methods.rb
@@ -0,0 +1,77 @@
+# frozen_string_literal: true
+
+require "active_support/core_ext/hash/except"
+require "active_support/core_ext/hash/slice"
+require "active_record/relation/merger"
+
+module ActiveRecord
+  module SpawnMethods
+    # This is overridden by Associations::CollectionProxy
+    def spawn #:nodoc:
+      @delegate_to_klass ? klass.all : clone
+    end
+
+    # Merges in the conditions from <tt>other</tt>, if <tt>other</tt> is an ActiveRecord::Relation.
+    # Returns an array representing the intersection of the resulting records with <tt>other</tt>, if <tt>other</tt> is an array.
+    #
+    #   Post.where(published: true).joins(:comments).merge( Comment.where(spam: false) )
+    #   # Performs a single join query with both where conditions.
+    #
+    #   recent_posts = Post.order('created_at DESC').first(5)
+    #   Post.where(published: true).merge(recent_posts)
+    #   # Returns the intersection of all published posts with the 5 most recently created posts.
+    #   # (This is just an example. You'd probably want to do this with a single query!)
+    #
+    # Procs will be evaluated by merge:
+    #
+    #   Post.where(published: true).merge(-> { joins(:comments) })
+    #   # => Post.where(published: true).joins(:comments)
+    #
+    # This is mainly intended for sharing common conditions between multiple associations.
+    def merge(other)
+      if other.is_a?(Array)
+        records & other
+      elsif other
+        spawn.merge!(other)
+      else
+        raise ArgumentError, "invalid argument: #{other.inspect}."
+      end
+    end
+
+    def merge!(other) # :nodoc:
+      if other.is_a?(Hash)
+        Relation::HashMerger.new(self, other).merge
+      elsif other.is_a?(Relation)
+        Relation::Merger.new(self, other).merge
+      elsif other.respond_to?(:to_proc)
+        instance_exec(&other)
+      else
+        raise ArgumentError, "#{other.inspect} is not an ActiveRecord::Relation"
+      end
+    end
+
+    # Removes from the query the condition(s) specified in +skips+.
+    #
+    #   Post.order('id asc').except(:order)                  # discards the order condition
+    #   Post.where('id > 10').order('id asc').except(:where) # discards the where condition but keeps the order
+    def except(*skips)
+      relation_with values.except(*skips)
+    end
+
+    # Removes any condition from the query other than the one(s) specified in +onlies+.
+    #
+    #   Post.order('id asc').only(:where)         # discards the order condition
+    #   Post.order('id asc').only(:where, :order) # uses the specified order
+    def only(*onlies)
+      relation_with values.slice(*onlies)
+    end
+
+    private
+
+      def relation_with(values)
+        result = Relation.create(klass, values: values)
+        result.extend(*extending_values) if extending_values.any?
+        result
+      end
+  end
+end