Don't require a DB connection when setting primary key.

Closes #2807.

activerecord/lib/active_record/attribute_methods/primary_key.rb

@@ -66,7 +66,6 @@ module ActiveRecord
           @primary_key ||= ''
           self.original_primary_key = @primary_key
           value &&= value.to_s
-          connection_pool.primary_keys[table_name] = value
           self.primary_key = block_given? ? instance_eval(&block) : value
         end
       end

activerecord/lib/active_record/base.rb

@@ -707,6 +707,10 @@ module ActiveRecord #:nodoc:
 
       # Returns an array of column objects for the table associated with this class.
       def columns
+        if defined?(@primary_key)
+          connection_pool.primary_keys[table_name] ||= primary_key
+        end
+
         connection_pool.columns[table_name]
       end
 

activerecord/test/cases/primary_keys_test.rb

@@ -145,4 +145,20 @@ class PrimaryKeysTest < ActiveRecord::TestCase
     k.set_primary_key "bar"
     assert_equal k.connection.quote_column_name("bar"), k.quoted_primary_key
   end
+
+  def test_set_primary_key_with_no_connection
+    return skip("disconnect wipes in-memory db") if in_memory_db?
+
+    connection = ActiveRecord::Base.remove_connection
+
+    model = Class.new(ActiveRecord::Base) do
+      set_primary_key 'foo'
+    end
+
+    assert_equal 'foo', model.primary_key
+
+    ActiveRecord::Base.establish_connection(connection)
+
+    assert_equal 'foo', model.primary_key
+  end
 end