Move SchemaMigration to migration_context

This PR moves the `schema_migration` to `migration_context` so that we
can access the `schema_migration` per connection.

This does not change behavior of the SchemaMigration if you are using
one database. This also does not change behavior of any public APIs.
`Migrator` is private as is `MigrationContext` so we can change these as
needed.

We now need to pass a `schema_migration` to `Migrator` so that we can
run migrations on the right connection outside the context of a rake
task.

The bugs this fixes were discovered while debugging the issues around
the SchemaCache on initialization with multiple database. It was clear
that `get_all_versions` wouldn't work without these changes outside the
context of a rake task (because in the rake task we establish a
connection and change AR::Base.connection to the db we're running on).

Because the `SchemaCache` relies on the `SchemaMigration` information we
need to make sure we store it per-connection rather than on
ActiveRecord::Base.

[Eileen M. Uchitelle & Aaron Patterson]

1 files changed, 4 insertions, 3 deletions

diff --git a/railties/test/application/loading_test.rb b/railties/test/application/loading_test.rb
index c262a6435e..9ab400acfc 100644
--- a/railties/test/application/loading_test.rb
+++ b/railties/test/application/loading_test.rb
@@ -133,11 +133,12 @@ class LoadingTest < ActiveSupport::TestCase
     require "#{rails_root}/config/environment"
     setup_ar!
 
-    assert_equal [ActiveStorage::Blob, ActiveStorage::Attachment, ActiveRecord::SchemaMigration, ActiveRecord::InternalMetadata, ApplicationRecord].collect(&:to_s).sort, ActiveRecord::Base.descendants.collect(&:to_s).sort
+    initial = [ActiveStorage::Blob, ActiveStorage::Attachment, ActiveRecord::SchemaMigration, ActiveRecord::InternalMetadata, ApplicationRecord, "primary::SchemaMigration"].collect(&:to_s).sort
+    assert_equal initial, ActiveRecord::Base.descendants.collect(&:to_s).sort
     get "/load"
-    assert_equal [ActiveStorage::Blob, ActiveStorage::Attachment, ActiveRecord::SchemaMigration, ActiveRecord::InternalMetadata, ApplicationRecord, Post].collect(&:to_s).sort, ActiveRecord::Base.descendants.collect(&:to_s).sort
+    assert_equal [Post].collect(&:to_s).sort, ActiveRecord::Base.descendants.collect(&:to_s).sort - initial
     get "/unload"
-    assert_equal [ActiveStorage::Blob, ActiveStorage::Attachment, ActiveRecord::SchemaMigration, ActiveRecord::InternalMetadata].collect(&:to_s).sort, ActiveRecord::Base.descendants.collect(&:to_s).sort
+    assert_equal ["ActiveRecord::InternalMetadata", "ActiveRecord::SchemaMigration", "primary::SchemaMigration"], ActiveRecord::Base.descendants.collect(&:to_s).sort.uniq
   end
 
   test "initialize cant be called twice" do