Retrieving data from third model in database using primary keys

Question

I would like to access the current user's albums by using the primary keys in the Schema below. I have User, Band, And Album models as follows...

class User < ActiveRecord::Base
  devise :database_authenticatable, :registerable,
     :recoverable, :rememberable, :trackable, :validatable

  has_many :bands
end

class Band < ActiveRecord::Base
  has_many :albums
  has_many :users
end

class Album < ActiveRecord::Base
  belongs_to :band
end

Schema as follows...

create_table "albums", force: true do |t|
  t.string   "name"
  t.string   "releaseDate"
  t.string   "artWorkUrl"
  t.integer  "band_id"
  t.datetime "created_at"
  t.datetime "updated_at"
end

create_table "bands", force: true do |t|
  t.string   "name"
  t.datetime "created_at"
  t.datetime "updated_at"
  t.integer  "user_id"
end

create_table "users", force: true do |t|
  t.string   "email",                  default: "", null: false
  t.string   "encrypted_password",     default: "", null: false
  t.string   "reset_password_token"
  t.datetime "reset_password_sent_at"
  t.datetime "remember_created_at"
  t.integer  "sign_in_count",          default: 0,  null: false
  t.datetime "current_sign_in_at"
  t.datetime "last_sign_in_at"
  t.string   "current_sign_in_ip"
  t.string   "last_sign_in_ip"
  t.datetime "created_at"
  t.datetime "updated_at"
end

and am trying to use the following method in the Albums controller...

def albums
  @albums = current_user.bands.albums
end

Sorry, i'm sure this is a noob question. I know this should be a simple primary key access through the user --> bands --> albums, using user_id and band_id but have been unable to populate the current users albums through bands. Any insight in much appreciated.

score 1 · Accepted Answer · edited May 23 '17 at 12:05

1

def albums
  band_ids = current_user.bands.map(&:id)
  @albums = Album.where(band_id: band_ids)
end

By separating the queries you do not have the n+1 problem: What is SELECT N+1?

edited May 23 '17 at 12:05

Community

1
1

answered May 28 '14 at 05:52

aledalgrande

5,167
3
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That's great - thanks, the n+1 explanation was enlightening as well. – LDro May 28 '14 at 06:15
First row: "I would like to access the current user's albums" – aledalgrande May 28 '14 at 06:16
Okay - I was looking at the associations. Mind if I post the answer that I was working on? – Richard Peck May 28 '14 at 06:16
I think SO would benefit from more answers. – aledalgrande May 28 '14 at 06:18
:) I posted anyway lol. I agree with your `n+1` statement but would prefer to stick to convention – Richard Peck May 28 '14 at 06:20

score 1 · Answer 2 · answered May 28 '14 at 06:17

by using the primary keys

You should be looking at the foreign_keys (primary_keys are when you reference the same table). So what you're really asking should be how do I set up the Rails associations correctly for my models?

Here's how:

#app/models/user.rb
Class User < ActiveRecord::Base
   has_many :bands
end

#app/models/band.rb
Class Band < ActiveRecord::Base
    has_many :albums
    belongs_to :user
end

#app/models/album.rb
Class Album < ActiveRecord::Base
    belongs_to :band
end

This will allow you to call current_user.bands.first.albums

--

Notes

You have to remember the bands association will be a collection. This means you can't just call ....bands.albums. Instead, you'll need to loop through the array like this:

<% for band in current_user.bands do %>
   <%= band.albums.inspect %>
<% end %>

You're right - I'm slowly learning (been at ruby/rails for about 6 weeks or so) that I need to think harder about architecting my models/association before jumping into a project. Thanks for taking the time to answer the question. — LDro, May 28 '14 at 06:30
No problem! Rails is amazing - congrats on sticking with for 6 weeks!! What is your goal with it? — Richard Peck, May 28 '14 at 06:31

Rajdeep Singh · Answer 3 · 2014-05-28T06:06:25.970

0

In user class define one more association

class User < ActiveRecord::Base    
  has_many :albums, :through => bands
end

In your controller

@albums = current_user.albums

edited May 28 '14 at 06:06

answered May 28 '14 at 05:57

Rajdeep Singh

17,621
6
53
78

This will not work, `band` does not act as the join table in this case. – aledalgrande May 28 '14 at 06:17
that's why I've added `has_many :albums, :through => bands` to user model, worth giving a try ;) – Rajdeep Singh May 28 '14 at 06:19

Retrieving data from third model in database using primary keys

3 Answers3