Move where merging logic over to `WhereClause`

This object being a black box, it knows the details of how to merge
itself with another where clause. This removes all references to where
values or bind values in `Relation::Merger`

1 files changed, 35 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/relation/where_clause.rb b/activerecord/lib/active_record/relation/where_clause.rb
index b39fc1fed1..cd6da052a9 100644
--- a/activerecord/lib/active_record/relation/where_clause.rb
+++ b/activerecord/lib/active_record/relation/where_clause.rb
@@ -1,6 +1,6 @@
 module ActiveRecord
   class Relation
-    class WhereClause
+    class WhereClause # :nodoc:
       attr_reader :parts, :binds
 
       def initialize(parts, binds)
@@ -15,6 +15,13 @@ module ActiveRecord
         )
       end
 
+      def merge(other)
+        WhereClause.new(
+          parts_unreferenced_by(other) + other.parts,
+          non_conflicting_binds(other) + other.binds,
+        )
+      end
+
       def ==(other)
         other.is_a?(WhereClause) &&
           parts == other.parts &&
@@ -24,6 +31,33 @@ module ActiveRecord
       def self.empty
         new([], [])
       end
+
+      protected
+
+      def referenced_columns
+        @referenced_columns ||= begin
+          equality_nodes = parts.select { |n| equality_node?(n) }
+          Set.new(equality_nodes, &:left)
+        end
+      end
+
+      private
+
+      def parts_unreferenced_by(other)
+        parts.reject do |n|
+          equality_node?(n) && other.referenced_columns.include?(n.left)
+        end
+      end
+
+      def equality_node?(node)
+        node.respond_to?(:operator) && node.operator == :==
+      end
+
+      def non_conflicting_binds(other)
+        conflicts = referenced_columns & other.referenced_columns
+        conflicts.map! { |node| node.name.to_s }
+        binds.reject { |col, _| conflicts.include?(col.name) }
+      end
     end
   end
 end