mysql hack: set length for binary indexes

config/initializers/mysql_set_length_for_binary_indexes.rb

@@ -0,0 +1,25 @@
+# This patches ActiveRecord so indexes for binary columns created using the
+# MySQL adapter apply a length of 20. Otherwise MySQL can't create an index on
+# binary columns.
+
+if defined?(ActiveRecord::ConnectionAdapters::Mysql2Adapter)
+  module ActiveRecord
+    module ConnectionAdapters
+      class Mysql2Adapter < AbstractMysqlAdapter
+        alias_method :__gitlab_add_index2, :add_index
+
+        def add_index(table_name, column_names, options = {})
+          Array(column_names).each do |column_name|
+            column = ActiveRecord::Base.connection.columns(table_name).find { |c| c.name == column_name }
+
+            if column&.type == :binary
+              options[:length] = 20
+            end
+          end
+
+          __gitlab_add_index2(table_name, column_names, options)
+        end
+      end
+    end
+  end
+end