How to implement complex SQL queries in Ruby on Rails? - sql

Question

I have four tables: "users", "user_groups", "groups", "categories".
"users" and "groups" are many_to_many relations through "user_groups".
"groups" and "categories" are many_to_one relations.
I created the following SQL query, but I'm not sure how to implement it in Ruby on Rails:
SELECT u.*
FROM users u
WHERE EXISTS (SELECT 1
FROM user_groups ug,
groups g,
categories c
WHERE u.id = ug.user_id
AND ug.group_id = g.id
AND g.category_id = c.id
AND c.id in ('1, 2, 3'))
What is the best way to implement it without using raw SQL in Ruby on Rails?

Answer 1

I am not a big fan of always translating complex queries into the ActiveRecord query language. Instead, I think it is perfectly fine to write complex queries in plain SQL because SQL is usually easier to write and to understand.
That said, I think this might work:
User.where(
id: UserGroup.select('user_groups.user_id')
.joins(groups: :categories)
.where(categories: { id: [1, 2, 3] })
)

Answer 2

This is probably a lot less complicated than you think:
class User
has_many :user_groups
has_many :groups, through: :user_groups
end
class Group
has_many :user_groups
has_many :users, through: :user_groups
belongs_to :category
end
User.joins(:groups)
.where(groups: { category_id: [1,2,3] })
ActiveRecord will handle joining the intermediate table.
Since this is a left inner join any rows without a match in the join table are omitted. No need to do WHERE EXISTS .... This will give you users that belong to at least one group with the category 1, 2 or 3.