At the time of writing, this is how the `TopicPosterSerializer` looks
like:
```
class TopicPosterSerializer < ApplicationSerializer
attributes :extras, :description
has_one :user, serializer: PosterSerializer
has_one :primary_group, serializer: PrimaryGroupSerializer
has_one :flair_group, serializer: FlairGroupSerializer
end
```
Within `PosterSerializer`, the `primary_group` and `flair_group`
association is requested on the `user` object. However, the associations
have not been loaded on the `user` object at this point leading to the
N+1 queries problem. One may wonder
why the associations have not been loaded when the `TopicPosterSerializer`
has `has_one :primary_group` and `has_one :flair_group`. It turns out that `TopicPoster`
is just a struct containing the `user`, `primary_group` and
`flair_group` objects. The `primary_group` and `flair_group`
ActiveRecord objects are loaded seperately in `UserLookup` and not preloaded when querying for
the users. This is done for performance reason so that we are able to
load the `primary_group` and `flair_group` records in a single query
without duplication.
User flair was given by user's primary group. This PR separates the
two, adds a new field to the user model for flair group ID and users
can select their flair from user preferences now.
Alan Guo Xiang Tan
Arpit Jalan
Jarek Radosz
Bianca Nenciu
Neil Lalonde
David Taylor