* Make Array-creation behavior of Paginable more predictable
Paginable.paginate_by_id usually returns ActiveRecord::Relation, but it
returns an Array if min_id option is present. The behavior caused problems
fixed with the following commits:
- 552e886b64
- b63ede5005
- 64ef37b89d
To prevent from recurring similar problems, this commit introduces two
changes:
- The scope now always returns an Array whether min_id option is present
or not.
- The scope is renamed to to_a_paginated_by_id to clarify it returns an
Array.
* Transform Paginable.to_a_paginated_by_id from a scope to a class method
https://api.rubyonrails.org/classes/ActiveRecord/Scoping/Named/ClassMethods.html#method-i-scope
> The method is intended to return an ActiveRecord::Relation object, which
> is composable with other scopes.
Paginable.to_a_paginated_by_id returns an Array and is not appropriate
as a scope.
* Replace incorrect use of distinct with group
Some uses of ActiveRecord::QueryMethods#distinct pass field names but they
are incorrect for the current version of Rails.
ActiveRecord::QueryMethods#group provides the expected behavior and
benefits performance. See commit 6da24aad4cafdef8d8a2c92bac2002a5fc2fe9c8.
* Introduce ApplicationController#cache_collection_paginated_by_id
ApplicationController#cache_collection_paginated_by_id fuses
ApplicationController#cache_collection and Paginable.paginate_by_id.
An advantage of this method is that it prevents from modifying scope which
Paginable.paginate_by_id may provide.
ApplicationController#cache_collection always return an array and there
is no possibility of the scope modification. It is also clear for a
programmer, considering the implication of "cache".
This method can also emit more efficient queries by using
Cacheable.cache_ids before calling Paginable.paginate_by_id.
Some uses of ActiveRecord::QueryMethods#distinct pass field names but they
are incorrect for the current version of Rails.
ActiveRecord::QueryMethods#group provides the expected behavior and
benefits performance. See commit 6da24aad4cafdef8d8a2c92bac2002a5fc2fe9c8.
The old implementation had two queries:
1. The query constructed in Api::V1::FavouritesController#results
2. The query constructed in #cached_favourites, which is merged with 1.
Both of them are issued againt PostgreSQL. The combination of the two
queries caused the following problems:
- The small window between the two queries involves race conditions.
- Minor performance inefficiency.
Moreover, the construction of query 2, which involves merging with query
1 has a bug. Query 1 is finalized with paginate_by_id, but paginate_by_id
returns an array when min_id parameter is specified. The behavior prevents
from merging the query, and in the real world, ActiveRecord simply ignores
the merge (!), which results in querying the entire scan of statuses and
favourites table.
This change fixes these issues by simply letting query 1 get all the works
done.
DISTINCT clause removes duplicated records according to all the selected
attributes. In reality, it can remove duplicated records only looking at
statuses.id, but the clause confuses the query planner and yields
insufficient performance.
The behavior is also problematic if the scope produced by HashQueryService
is used to query columns without id (using pluck method, for example). The
scope is expected to contain unique statuses, but the uniquness will be
evaluated with some arbitrary columns other than id.
GROUP BY clause resolves those problem by explicitly specifying the
column to take into account for the record distinction.
A workaround for the problem of DISTINCT clause in
Api::V1::Timelines::TagController is no longer necessary and removed.
Akihiko Odaki