Make sure we reset the connection_specification_name on

remove_connection

When calling `remove_connection` on a model, we delete the pool so we also
need to reset the `connection_specification_name` so it will fallback to
the parent.
This was the current behavior before rails 5, which will fallback to the
parent connection pool.

[fixes #24959]

Special thanks to @jrafanie for working with me on this fix.

activerecord/lib/active_record/connection_handling.rb

@@ -95,7 +95,7 @@ def connection
 
     # Return the specification name from the current class or its parent.
     def connection_specification_name
-      unless defined?(@connection_specification_name)
+      if !defined?(@connection_specification_name) || @connection_specification_name.nil?
         @connection_specification_name = self == Base ? "primary" : superclass.connection_specification_name
       end
       @connection_specification_name
@@ -133,6 +133,13 @@ def connected?
     end
 
     def remove_connection(name = connection_specification_name)
+      # if removing a connection that have a pool, we reset the
+      # connection_specification_name so it will use the parent
+      # pool.
+      if connection_handler.retrieve_connection_pool(name)
+        self.connection_specification_name = nil
+      end
+
       connection_handler.remove_connection(name)
     end
 

activerecord/test/cases/connection_adapters/connection_handler_test.rb

@@ -89,6 +89,20 @@ def test_retrieve_connection_pool_copies_schema_cache_from_ancestor_pool
           assert_equal @pool.schema_cache.size, Marshal.load(rd.read)
           rd.close
         end
+
+        def test_a_class_using_custom_pool_and_switching_back_to_primary
+          klass2     = Class.new(Base)   { def self.name; 'klass2'; end }
+
+          assert_equal klass2.connection.object_id, ActiveRecord::Base.connection.object_id
+
+          pool = klass2.establish_connection(ActiveRecord::Base.connection_pool.spec.config)
+          assert_equal klass2.connection.object_id, pool.connection.object_id
+          refute_equal klass2.connection.object_id, ActiveRecord::Base.connection.object_id
+
+          klass2.remove_connection
+
+          assert_equal klass2.connection.object_id, ActiveRecord::Base.connection.object_id
+        end
       end
     end
   end