allow override of uuid_generate_v4() default by passing default: nil

without this, it's not possible to use UUID primary keys without uuid-ossp installed and activated

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -330,9 +330,38 @@ def json(name, options = {})
       class TableDefinition < ActiveRecord::ConnectionAdapters::TableDefinition
         include ColumnMethods
 
+        # Defines the primary key field.
+        # Use of the native PostgreSQL UUID type is supported, and can be used
+        # by defining your tables as such:
+        #
+        #   create_table :stuffs, id: :uuid do |t|
+        #     t.string :content
+        #     t.timestamps
+        #   end
+        #
+        # By default, this will use the +uuid_generate_v4()+ function from the
+        # +uuid-ossp+ extension, which MUST be enabled on your databse.  To enable
+        # the +uuid-ossp+ extension, you can use the +enable_extension+ method in your
+        # migrations To use a UUID primary key without +uuid-ossp+ enabled, you can
+        # set the +:default+ option to nil:
+        #
+        #   create_table :stuffs, id: false do |t|
+        #     t.primary_key :id, :uuid, default: nil
+        #     t.uuid :foo_id
+        #     t.timestamps
+        #   end
+        #
+        # You may also pass a different UUID generation function from +uuid-ossp+
+        # or another library.
+        #
+        # Note that setting the UUID primary key default value to +nil+
+        # will require you to assure that you always provide a UUID value
+        # before saving a record (as primary keys cannot be nil).  This might be
+        # done via the SecureRandom.uuid method and a +before_save+ callback,
+        # for instance.
         def primary_key(name, type = :primary_key, options = {})
           return super unless type == :uuid
-          options[:default] ||= 'uuid_generate_v4()'
+          options[:default] = options.fetch(:default, 'uuid_generate_v4()')
           options[:primary_key] = true
           column name, type, options
         end

activerecord/test/cases/adapters/postgresql/uuid_test.rb

@@ -51,3 +51,34 @@ def test_auto_create_uuid
     assert_not_nil u.other_uuid
   end
 end
+
+class PostgresqlUUIDTestNilDefault < ActiveRecord::TestCase
+  class UUID < ActiveRecord::Base
+    self.table_name = 'pg_uuids'
+  end
+
+  def setup
+    @connection = ActiveRecord::Base.connection
+
+    @connection.reconnect!
+
+    @connection.transaction do
+      @connection.create_table('pg_uuids', id: false) do |t|
+        t.primary_key :id, :uuid, default: nil
+        t.string 'name'
+      end
+    end
+  end
+
+  def teardown
+    @connection.execute 'drop table if exists pg_uuids'
+  end
+
+  def test_id_allows_default_override_via_nil
+    col_desc = @connection.execute("SELECT pg_get_expr(d.adbin, d.adrelid) as default
+                                    FROM pg_attribute a
+                                    LEFT JOIN pg_attrdef d ON a.attrelid = d.adrelid AND a.attnum = d.adnum
+                                    WHERE a.attname='id' AND a.attrelid = 'pg_uuids'::regclass").first
+    assert_nil col_desc["default"]
+  end
+end