Relationship Models

Question

I'm fairly new to rails and I'm having trouble with some Model Relationships.

I have these tables

+--------------+-----------------+------+
| Artists      | Artist Features | Tags |
+--------------+-----------------+------+
| ID           | ID              | ID   |
| Name         | artist_id       | name |
| Address      | price           |      |
| Country      | tag_id          |      |
| Email        |                 |      |
+--------------+-----------------+------+

I have relationships between Artists and Artist Features artists has_many artist_features and artist_features has_many tags. I want to, with a query get all Artists, features and tags based on the structure above, artist_id and tag_id

I've used the below to get the artists and the corresponding artist_features but would like to also get the tags too based on the tag_id

@Artists = Artist.all.artist_features

Answer 1

If an artist feature has_many tags, then you need to change your table columns to match this relationship structure.

A has_many / belongs_to relationship always requires that the model with the belongs_to method call has a foreign key column.

In this case, you want to have a set of tags and allow artist features to reference these tags. We can do this by adding another model between ArtistFeature and Tag , called ArtistFeatureTag , with a table artist_feature_tags .

Your model structure then would look like this:

Artist: id , name , address , country , email

class Artist
  has_many :artist_features
end

Artist Feature: id , artist_id , price

class ArtistFeature
  belongs_to :artist
  has_many :artist_feature_tags
  has_many :tags, through: :artist_feature_tags
end

ArtistFeatureTag: id , artist_feature_id , tag_id

class ArtistFeatureTag
  belongs_to :artist_feature
  belongs_to :tag
end

Tag: id , artist_feature_id , name

class Tag
  has_many :artist_feature_tags
  has_many :artist_features, through: :artist_feature_tags
end

The extra table may seem unintuitive, but it's necessary to store the relationship between artist features and tags, ie multiple artist features may reference the same tag, and these references take the form of ArtistFeatureTag s.

Note that this is basically a has and belongs to many-type relationship , but I've decided to explicitly create a model for the relationship since I think it makes things more clear.