Compared to the merge_request_diff association:
1. It's simpler to query. The query uses a foreign key to the
merge_request_diffs table, so no ordering is necessary.
2. It's faster for preloading. The merge_request_diff association has to load
every diff for the MRs in the set, then discard all but the most recent for
each. This association means that Rails can just query for N diffs from N
MRs.
3. It's more complicated to update. This is a bidirectional foreign key, so we
need to update two tables when adding a diff record. This also means we need
to handle this as a special case when importing a GitLab project.
There is some juggling with this association in the merge request model:
* `MergeRequest#latest_merge_request_diff` is _always_ the latest diff.
* `MergeRequest#merge_request_diff` reuses
`MergeRequest#latest_merge_request_diff` unless:
* Arguments are passed. These are typically to force-reload the association.
* It doesn't exist. That means we might be trying to implicitly create a
diff. This only seems to happen in specs.
* The association is already loaded. This is important for the reasons
explained in the comment, which I'll reiterate here: if we a) load a
non-latest diff, then b) get its `merge_request`, then c) get that MR's
`merge_request_diff`, we should get the diff we loaded in c), even though
that's not the latest diff.
Basically, `MergeRequest#merge_request_diff` is the latest diff in most cases,
but not quite all.
also, I refactored the MergeRequest#fetch_ref method to express
the side-effect that this method has.
MergeRequest#fetch_ref -> MergeRequest#fetch_ref!
Repository#fetch_source_branch -> Repository#fetch_source_branch!
This finishes the procedure for migrating events from the old format
into the new format. Code no longer uses the old setup and the database
tables used during the migration process are swapped, with the old table
being dropped.
While the database migration can be reversed this will 1) take a lot of
time as data has to be coped around 2) won't restore data in the
"events.data" column as we have no way of restoring this.
Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/37241
James Lopez
Sean McGivern
Yorick Peterse
Shinya Maeda
James Lopez
Rémy Coutable
Alessio Caiazza
Eric Eastwood
Grzegorz Bizon
Markus Koller
micael.bergeron
Sean McGivern
Matt Coleman
Vlad
Bob Van Landuyt
Kamil Trzciński
Grzegorz Bizon
Valery Sizov
Jarka Kadlecova
Maxim Rydkin
Zeger-Jan van de Weg