• Sean McGivern's avatar
    Use latest_merge_request_diff association · 991bf24e
    Sean McGivern authored
    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.
    991bf24e
20171115164540_populate_merge_requests_latest_merge_request_diff_id_take_two.rb 840 Bytes