Rename base model to avoid Rails getting the wrong ApplicationRecord

We have accidentally caused all CI models to use Ci::ApplicationRecord
when we only want Ci::InstanceVariable to do so

app/models/ci/application_record.rb → app/models/ci/base_model.rb

@@ -7,7 +7,7 @@ module Ci
   # This class is part of a migration to move all CI classes to a new separate database.
   # Initially we are only going to be moving the `Ci::InstanceVariable` model and it will be duplicated in the main and CI tables
   # Do not extend this class in any other models.
-  class ApplicationRecord < ::ApplicationRecord
+  class BaseModel < ::ApplicationRecord
     self.abstract_class = true
 
     if Gitlab::Database.has_config?(:ci)

app/models/ci/instance_variable.rb

 # frozen_string_literal: true
 
 module Ci
-  class InstanceVariable < ::Ci::ApplicationRecord
+  class InstanceVariable < ::Ci::BaseModel
     extend Gitlab::Ci::Model
     extend Gitlab::ProcessMemoryCache::Helper
     include Ci::NewHasVariable

doc/development/database/multiple_databases.md

@@ -95,7 +95,7 @@ database. This is to match the default name Rails has.
 
 ### Migrations
 
-Any migrations that affect `Ci::ApplicationRecord` models
+Any migrations that affect `Ci::BaseModel` models
 and their tables must be placed in two directories for now:
 
 - `db/migrate`