rails group order by count - ruby-on-rails

Question

I have five tables: users, interests, animals, interests_animals, interests_users.
User
foo
Interest 1, 2, 3
Animal 1, 2, 3
foo has interests 1, 2, 3
Interest 1 has Animal 1, 2
Interest 2 has Animal 1, 2, 3
Interest 3 has Animal 3
I need return all animals through interests grouped for interest id of foo ordered by animals count
I trying like this:
SELECT animals.* FROM animals
INNER JOIN interests_animals ON animals.id = interests_animals.animal_id
INNER JOIN interests ON interests_animals.interest_id = interests.id
INNER JOIN interests_users ON interests.id = interests_users.interest_id
WHERE interests_users.user_id = XXX
GROUP BY animals.id
ORDER BY COUNT(interests_animals.animal_id);
I need that the animals are returned in orders 2, 1, 3, but always returning 1,2,3

Answer 1

You need explicitly specify column(s), on which you do GROUP BY in SELECT clause.
All other parts of SELECT clause must be aggregates like count(), sum(), etc.
Notice, that we use count(distinct ..) here because each animal ID might appear multiple times due to the chain of JOINs:
SELECT
interests.id,
COUNT(DISTINCT animals.id) as animals_count
JOIN interests_animals ON animals.id = interests_animals.animal_id
JOIN interests ON interests_animals.interest_id = interests.id
JOIN interests_users ON interests.id = interests_users.interest_id
WHERE interests_users.user_id = XXX
GROUP BY 1
ORDER BY 2 desc;
-- in GROUP BY and ORDER BY, it is usually convenient to use just numbers -- "1" means "the 1st column of SELECT clause", etc.
Also, "INNER" is an optional keyword (simply "JOIN" and "INNER JOIN" are the same thing).
Also, as a side note, you might found useful to add this to your SELECT clause:
, array_agg(animals.id order by animals.id) as animal_ids
-- this will give you integer array of all animal IDs that relate to a particular interest, ordered.