1 files changed, 83 insertions, 0 deletions
diff --git a/activerecord/lib/arel/visitors/mysql.rb b/activerecord/lib/arel/visitors/mysql.rb
new file mode 100644
index 0000000000..dd77cfdf66
--- /dev/null
+++ b/activerecord/lib/arel/visitors/mysql.rb
@@ -0,0 +1,83 @@
+# frozen_string_literal: true
+
+module Arel # :nodoc: all
+  module Visitors
+    class MySQL < Arel::Visitors::ToSql
+      private
+        def visit_Arel_Nodes_Bin(o, collector)
+          collector << "BINARY "
+          visit o.expr, collector
+        end
+
+        def visit_Arel_Nodes_UnqualifiedColumn(o, collector)
+          visit o.expr, collector
+        end
+
+        ###
+        # :'(
+        # http://dev.mysql.com/doc/refman/5.0/en/select.html#id3482214
+        def visit_Arel_Nodes_SelectStatement(o, collector)
+          if o.offset && !o.limit
+            o.limit = Arel::Nodes::Limit.new(18446744073709551615)
+          end
+          super
+        end
+
+        def visit_Arel_Nodes_SelectCore(o, collector)
+          o.froms ||= Arel.sql("DUAL")
+          super
+        end
+
+        def visit_Arel_Nodes_Concat(o, collector)
+          collector << " CONCAT("
+          visit o.left, collector
+          collector << ", "
+          visit o.right, collector
+          collector << ") "
+          collector
+        end
+
+        def visit_Arel_Nodes_IsNotDistinctFrom(o, collector)
+          collector = visit o.left, collector
+          collector << " <=> "
+          visit o.right, collector
+        end
+
+        def visit_Arel_Nodes_IsDistinctFrom(o, collector)
+          collector << "NOT "
+          visit_Arel_Nodes_IsNotDistinctFrom o, collector
+        end
+
+        # In the simple case, MySQL allows us to place JOINs directly into the UPDATE
+        # query. However, this does not allow for LIMIT, OFFSET and ORDER. To support
+        # these, we must use a subquery.
+        def prepare_update_statement(o)
+          if o.offset || has_join_sources?(o) && has_limit_or_offset_or_orders?(o)
+            super
+          else
+            o
+          end
+        end
+        alias :prepare_delete_statement :prepare_update_statement
+
+        # MySQL is too stupid to create a temporary table for use subquery, so we have
+        # to give it some prompting in the form of a subsubquery.
+        def build_subselect(key, o)
+          subselect = super
+
+          # Materialize subquery by adding distinct
+          # to work with MySQL 5.7.6 which sets optimizer_switch='derived_merge=on'
+          unless has_limit_or_offset_or_orders?(subselect)
+            core = subselect.cores.last
+            core.set_quantifier = Arel::Nodes::Distinct.new
+          end
+
+          Nodes::SelectStatement.new.tap do |stmt|
+            core = stmt.cores.last
+            core.froms = Nodes::Grouping.new(subselect).as("__active_record_temp")
+            core.projections = [Arel.sql(quote_column_name(key.name))]
+          end
+        end
+    end
+  end
+end