Rails - ActiveRecord - Find Common Group Based on Commonalities

Question

Let's suppose I have the following models:

Group
  has_many :users

User
  belongs_to :group

And I have the following tables:

   User_id                    Group
  -------------------------------------
    1                        Police
    2                        Fire
    3                        Military
    4                        Police
    5                        Police
    1                        Fire

Most users belong to a single group, but some users, like user_id: 1 belongs to multiple groups. (He belongs to Police and Fire .

My question is: If I am given two user_id's , (say 1 and 2 ), how would I query to find out the common group that these 2 users belong to (eg, in the case above, the query would return Fire .

Hope this makes sense.

Answer 1

First of all I would recommend using a has_and_belongs_to_many relationship between User and Group:

class Group
    has_and_belongs_to_many :users
end

class User
    has_and_belongs_to_many :users
end

You'd have tables like:

create_table "groups", force: true do |t|
    t.string name
end

create_table "users", force: true do |t|
    ...
end

create_table "groups_users", id: false, force: true do |t|
    t.integer "group_id"
    t.integer "user_id"
end

This would prevent the repetition you have going on in the group field in the groups table currently. It would also help avoid certain data entry errors (spelling errors) and make building a better UI easier. There are other ways to overcome these same hurdles, but they take much more effort than is saved by not following the convention usually.

Then you can get the intersection of the groups for two users (user_a and user_b) like:

user_a.groups & user_b.groups # => an array of the groups that they both belong to

(user_a.groups & user_b.groups).collect(&:name) # => an array of the names of the groups they both belong to.

Answer 2

这实际上取决于您的关联是如何设置的，但这是在一个查询中执行此操作的一种方法

user_a.groups.where(id: user_b.groups.select(:id))