Refactor build_relation in the uniqueness validator to avoid low level predicate construction

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -504,15 +504,17 @@ module ActiveRecord
         @connection
       end
 
-      def case_sensitive_comparison(table, attribute, column, value) # :nodoc:
-        table[attribute].eq(value)
+      def case_sensitive_comparison(attribute, value) # :nodoc:
+        attribute.eq(value)
       end
 
-      def case_insensitive_comparison(table, attribute, column, value) # :nodoc:
+      def case_insensitive_comparison(attribute, value) # :nodoc:
+        column = column_for_attribute(attribute)
+
         if can_perform_case_insensitive_comparison_for?(column)
-          table[attribute].lower.eq(table.lower(value))
+          attribute.lower.eq(attribute.relation.lower(value))
         else
-          table[attribute].eq(value)
+          attribute.eq(value)
         end
       end
 
@@ -659,6 +661,11 @@ module ActiveRecord
             raise(ActiveRecordError, "No such column: #{table_name}.#{column_name}")
         end
 
+        def column_for_attribute(attribute)
+          table_name = attribute.relation.name
+          schema_cache.columns_hash(table_name)[attribute.name.to_s]
+        end
+
         def collector
           if prepared_statements
             Arel::Collectors::Composite.new(

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -476,9 +476,11 @@ module ActiveRecord
         SQL
       end
 
-      def case_sensitive_comparison(table, attribute, column, value) # :nodoc:
+      def case_sensitive_comparison(attribute, value) # :nodoc:
+        column = column_for_attribute(attribute)
+
         if column.collation && !column.case_sensitive?
-          table[attribute].eq(Arel::Nodes::Bin.new(value))
+          attribute.eq(Arel::Nodes::Bin.new(value))
         else
           super
         end

activerecord/lib/active_record/validations/uniqueness.rb

@@ -61,28 +61,21 @@ module ActiveRecord
           value = value.attributes[reflection.klass.primary_key] unless value.nil?
         end
 
-        if value.nil?
-          return klass.unscoped.where!(attribute => value)
+        relation = klass.unscoped
+        comparison = relation.bind_attribute(attribute, value) do |attr, bind|
+          return relation.none! unless bind.boundable?
+
+          if bind.nil?
+            attr.eq(bind)
+          elsif options[:case_sensitive]
+            klass.connection.case_sensitive_comparison(attr, bind)
+          else
+            # will use SQL LOWER function before comparison, unless it detects a case insensitive collation
+            klass.connection.case_insensitive_comparison(attr, bind)
+          end
         end
 
-        # the attribute may be an aliased attribute
-        if klass.attribute_alias?(attribute)
-          attribute = klass.attribute_alias(attribute)
-        end
-
-        attribute_name = attribute.to_s
-        value = klass.predicate_builder.build_bind_attribute(attribute_name, value)
-
-        table = klass.arel_table
-        column = klass.columns_hash[attribute_name]
-
-        comparison = if !options[:case_sensitive]
-          # will use SQL LOWER function before comparison, unless it detects a case insensitive collation
-          klass.connection.case_insensitive_comparison(table, attribute, column, value)
-        else
-          klass.connection.case_sensitive_comparison(table, attribute, column, value)
-        end
-        klass.unscoped.where!(comparison)
+        relation.where!(comparison)
       end
 
       def scope_relation(record, relation)