ActiveRecord select with OR and exclusive limit - ruby-on-rails

Question

I have the need to query the database and retrieve the last 10 objects that are either active or declined. We use the following:
User.where(status: [:active, :declined]).limit(10)
Now we need to get the last 10 of each status (total of 20 users)
I've tried the following:
User.where(status: :active).limit(10).or(User.where(status: : declined).limit(10))
# SELECT "users".* FROM "users" WHERE ("users"."status" = $1 OR "users"."status" = $2) LIMIT $3
This does the same as the previous query and returns only 10 users, of mixed statuses.
How can I get the last 10 active users and the last 10 declined users with a single query?

Answer 1

I'm not sure that SQL allows doing what you want. First thing I would try would be to use a subquery, something like this:
class User < ApplicationRecord
scope :active, -> { where status: :active }
scope :declined, -> { where status: :declined }
scope :last_active_or_declined, -> {
where(id: active.limit(10).pluck(:id))
.or(where(id: declined.limit(10).pluck(:id))
}
end
Then somewhere else you could just do
User.last_active_or_declined()
What this does is to perform 2 different subqueries asking separately for each of the group of users and then getting the ones in the propper group ids. I would say you could even forget about the pluck(:id) parts since ActiveRecord is smart enough to add the proper select clause to your SQL, but I'm not 100% sure and I don't have any Rails project at hand where I can try this.

Answer 2

limit is not a permitted value for #or relationship. If you check the Rails code, the Error raised come from here:
def or!(other) # :nodoc:
incompatible_values = structurally_incompatible_values_for_or(other)
unless incompatible_values.empty?
raise ArgumentError, "Relation passed to #or must be structurally compatible. Incompatible values: #{incompatible_values}"
end
# more code
end
You can check which methods are restricted further down in the code here:
STRUCTURAL_OR_METHODS = Relation::VALUE_METHODS - [:extending, :where, :having, :unscope, :references]
def structurally_incompatible_values_for_or(other)
STRUCTURAL_OR_METHODS.reject do |method|
get_value(method) == other.get_value(method)
end
end
You can see in the Relation class here that limit is restricted:
SINGLE_VALUE_METHODS = [:limit, :offset, :lock, :readonly, :reordering,
:reverse_order, :distinct, :create_with, :skip_query_cache,
:skip_preloading]
So you will have to resort to raw SQL I'm afraid

Answer 3

I don't think you can do it with a single query, but you can do it with two queries, get the record ids, and then build a query using those record ids.
It's not ideal but as you're just plucking ids the impact isn't too bad.
user_ids = User.where(status: :active).limit(10).pluck(:id) + User.where(status: :declined).limit(10).pluck(id)
users = User.where(id: user_ids)

Answer 4

I think you can use UNION. Install active_record_union and replace or with union:
User.where(status: :active).limit(10).union(User.where(status: :declined).limit(10))