Use type column first in multi-column indexes

`add_reference` can very helpfully add a multi-column index when you use
it to add a polymorphic reference. However, the first column in the
index is the `id` column, which is less than ideal.

The [PostgreSQL docs][1] say:
> A multicolumn B-tree index can be used with query conditions that
> involve any subset of the index's columns, but the index is most
> efficient when there are constraints on the leading (leftmost)
> columns.

The [MySQL docs][2] say:
> MySQL can use multiple-column indexes for queries that test all the
> columns in the index, or queries that test just the first column, the
> first two columns, the first three columns, and so on. If you specify
> the columns in the right order in the index definition, a single
> composite index can speed up several kinds of queries on the same
> table.

In a polymorphic relationship, the type column is much more likely to be
useful as the first column in an index than the id column. That is, I'm
more likely to query on type without an id than I am to query on id
without a type.

[1]: http://www.postgresql.org/docs/9.3/static/indexes-multicolumn.html
[2]: http://dev.mysql.com/doc/refman/5.0/en/multiple-column-indexes.html

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Use type column first in multi-column indexes created with `add-reference`
+
+    *Derek Prior*
+
 *   `AR::UnknownAttributeError` now includes the class name of a record.
 
         User.new(name: "Yuki Nishijima", project_attributes: {name: "kaminari"})

activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb

@@ -312,7 +312,7 @@ def references(*args)
         args.each do |col|
           column("#{col}_id", type, options)
           column("#{col}_type", :string, polymorphic.is_a?(Hash) ? polymorphic : options) if polymorphic
-          index(polymorphic ? %w(id type).map { |t| "#{col}_#{t}" } : "#{col}_id", index_options.is_a?(Hash) ? index_options : {}) if index_options
+          index(polymorphic ? %w(type id).map { |t| "#{col}_#{t}" } : "#{col}_id", index_options.is_a?(Hash) ? index_options : {}) if index_options
         end
       end
       alias :belongs_to :references

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -629,7 +629,7 @@ def add_reference(table_name, ref_name, options = {})
         type = options.delete(:type) || :integer
         add_column(table_name, "#{ref_name}_id", type, options)
         add_column(table_name, "#{ref_name}_type", :string, polymorphic.is_a?(Hash) ? polymorphic : options) if polymorphic
-        add_index(table_name, polymorphic ? %w[id type].map{ |t| "#{ref_name}_#{t}" } : "#{ref_name}_id", index_options.is_a?(Hash) ? index_options : {}) if index_options
+        add_index(table_name, polymorphic ? %w[type id].map{ |t| "#{ref_name}_#{t}" } : "#{ref_name}_id", index_options.is_a?(Hash) ? index_options : {}) if index_options
       end
       alias :add_belongs_to :add_reference
 

activerecord/test/cases/migration/references_index_test.rb

@@ -55,7 +55,7 @@ def test_creates_polymorphic_index
             t.references :foo, :polymorphic => true, :index => true
           end
 
-          assert connection.index_exists?(table_name, [:foo_id, :foo_type], :name => :index_testings_on_foo_id_and_foo_type)
+          assert connection.index_exists?(table_name, [:foo_type, :foo_id], name: :index_testings_on_foo_type_and_foo_id)
         end
       end
 
@@ -93,7 +93,7 @@ def test_creates_polymorphic_index_for_existing_table
             t.references :foo, :polymorphic => true, :index => true
           end
 
-          assert connection.index_exists?(table_name, [:foo_id, :foo_type], :name => :index_testings_on_foo_id_and_foo_type)
+          assert connection.index_exists?(table_name, [:foo_type, :foo_id], name: :index_testings_on_foo_type_and_foo_id)
         end
       end
     end

activerecord/test/cases/migration/references_statements_test.rb

@@ -42,7 +42,7 @@ def test_does_not_create_reference_id_index
 
       def test_creates_polymorphic_index
         add_reference table_name, :taggable, polymorphic: true, index: true
-        assert index_exists?(table_name, [:taggable_id, :taggable_type])
+        assert index_exists?(table_name, [:taggable_type, :taggable_id])
       end
 
       def test_creates_reference_type_column_with_default