Preload associations when exporting CI pipeline's builds

When exporting the CI pipelines, not all associations are preloaded, which causes N+1 queries causing the CI pipelines to take a long time to export.

The main problem is the statuses association, which is a polymorphic association, and each "real" association implements different sub-associations which don't allow us to preload them.

For instance, CommitStatus is the base class. The child classes are Ci::Build, Ci::Bridge, and GenericCommitStatus, and the last doesn't have the association metadata. Therefore when we export CommitStatus (statuses), which incorporates all child classes, the metadata can't be preloaded as the association doesn't exist for GenericCommitStatus

Solution

• Export each child class as a separate association instead of a unique association called statuses
• Preload the correct association for each child class
• Implement a solution that is compatible with previous exported files