1 files changed, 112 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/relation/merger.rb b/activerecord/lib/active_record/relation/merger.rb
new file mode 100644
index 0000000000..1c2a06328f
--- /dev/null
+++ b/activerecord/lib/active_record/relation/merger.rb
@@ -0,0 +1,112 @@
+require 'active_support/core_ext/object/blank'
+require 'active_support/core_ext/hash/keys'
+
+module ActiveRecord
+  class Relation
+    class Merger
+      attr_reader :relation, :other
+
+      def initialize(relation, other)
+        @relation = relation
+
+        if other.default_scoped? && other.klass != relation.klass
+          @other = other.with_default_scope
+        else
+          @other = other
+        end
+      end
+
+      def merge
+        HashMerger.new(relation, other.values).merge
+      end
+    end
+
+    class HashMerger
+      attr_reader :relation, :values
+
+      def initialize(relation, values)
+        values.assert_valid_keys(*Relation::VALUE_METHODS)
+
+        @relation = relation
+        @values   = values
+      end
+
+      def normal_values
+        Relation::SINGLE_VALUE_METHODS +
+          Relation::MULTI_VALUE_METHODS -
+          [:where, :order, :bind, :reverse_order, :lock, :create_with, :reordering]
+      end
+
+      def merge
+        normal_values.each do |name|
+          value = values[name]
+          relation.send("#{name}!", value) unless value.blank?
+        end
+
+        merge_multi_values
+        merge_single_values
+
+        relation
+      end
+
+      private
+
+      def merge_multi_values
+        relation.where_values = merged_wheres
+        relation.bind_values  = merged_binds
+
+        if values[:reordering]
+          # override any order specified in the original relation
+          relation.reorder! values[:order]
+        elsif values[:order]
+          # merge in order_values from r
+          relation.order! values[:order]
+        end
+
+        relation.extend(*values[:extending]) unless values[:extending].blank?
+      end
+
+      def merge_single_values
+        relation.lock_value          = values[:lock] unless relation.lock_value
+        relation.reverse_order_value = values[:reverse_order]
+
+        unless values[:create_with].blank?
+          relation.create_with_value = (relation.create_with_value || {}).merge(values[:create_with])
+        end
+      end
+
+      def merged_binds
+        if values[:bind]
+          (relation.bind_values + values[:bind]).uniq(&:first)
+        else
+          relation.bind_values
+        end
+      end
+
+      def merged_wheres
+        if values[:where]
+          merged_wheres = relation.where_values + values[:where]
+
+          unless relation.where_values.empty?
+            # Remove duplicates, last one wins.
+            seen = Hash.new { |h,table| h[table] = {} }
+            merged_wheres = merged_wheres.reverse.reject { |w|
+              nuke = false
+              if w.respond_to?(:operator) && w.operator == :==
+                name              = w.left.name
+                table             = w.left.relation.name
+                nuke              = seen[table][name]
+                seen[table][name] = true
+              end
+              nuke
+            }.reverse
+          end
+
+          merged_wheres
+        else
+          relation.where_values
+        end
+      end
+    end
+  end
+end