Add support for specifying comments for tables, columns, and indexes.

Comments are specified in migrations, stored in database itself (in its schema),
and dumped into db/schema.rb file.

This allows to generate good documentation and explain columns and tables' purpose
to everyone from new developers to database administrators.

For PostgreSQL and MySQL only. SQLite does not support comments at the moment.

See docs for PostgreSQL: http://www.postgresql.org/docs/current/static/sql-comment.html
See docs for MySQL: http://dev.mysql.com/doc/refman/5.7/en/create-table.html

1 files changed, 9 insertions, 4 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
index 6497b1cc31..cf04f32206 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
@@ -159,6 +159,10 @@ module ActiveRecord
         postgresql_version >= 90200
       end
 
+      def supports_comments?
+        true
+      end
+
       def index_algorithms
         { concurrently: 'CONCURRENTLY' }
       end
@@ -712,7 +716,7 @@ module ActiveRecord
         # Returns the list of a table's column names, data types, and default values.
         #
         # The underlying query is roughly:
-        #  SELECT column.name, column.type, default.value
+        #  SELECT column.name, column.type, default.value, column.comment
         #    FROM column LEFT JOIN default
         #      ON column.table_id = default.table_id
         #     AND column.num = default.column_num
@@ -732,7 +736,8 @@ module ActiveRecord
               SELECT a.attname, format_type(a.atttypid, a.atttypmod),
                      pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod,
              (SELECT c.collname FROM pg_collation c, pg_type t
-               WHERE c.oid = a.attcollation AND t.oid = a.atttypid AND a.attcollation <> t.typcollation)
+               WHERE c.oid = a.attcollation AND t.oid = a.atttypid AND a.attcollation <> t.typcollation),
+                     col_description(a.attrelid, a.attnum) AS comment
                 FROM pg_attribute a LEFT JOIN pg_attrdef d
                   ON a.attrelid = d.adrelid AND a.attnum = d.adnum
                WHERE a.attrelid = '#{quote_table_name(table_name)}'::regclass
@@ -746,8 +751,8 @@ module ActiveRecord
           $1.strip if $1
         end
 
-        def create_table_definition(name, temporary = false, options = nil, as = nil) # :nodoc:
-          PostgreSQL::TableDefinition.new(name, temporary, options, as)
+        def create_table_definition(name, temporary = false, options = nil, as = nil, comment = nil) # :nodoc:
+          PostgreSQL::TableDefinition.new(name, temporary, options, as, comment)
         end
 
         def can_perform_case_insensitive_comparison_for?(column)