Rails 4 belongs_to store ids in array column

Question

I have a Topic model that belongs to Category and Level.

Knowing that each Topic belongs to maximum 5-6 Level and 2-3 categories, could the category ids and the level ids of each topic be stored in 2 columns using array:true of Postgresql or is it bad practice to do so?

yez · Accepted Answer · 2015-12-14 23:00:34Z

1

Would a has_and_belongs_to_many relationship work for you?

class Topic
  has_and_belongs_to_many :levels
  has_and_belongs_to_many :categories
end

class Category
  has_and_belongs_to_many :topics
end

class Level
  has_and_belongs_to_many :topics
end

create_table :categories_topics do |t|
  t.integer :topic_id
  t.integer :category_id
end

create_table :levels_topics do |t|
  t.integer :level_id
  t.integer :topic_id
end

This would make the structure look like:

|--------|      |--------------|
| Topics | ---> | LevelsTopics |
|--------|      |--------------|
                    ^
|--------|          |
| Levels | ---------|
|--------|


|--------|      |-------------------|
| Topics | ---> | CategoriesTopics  |
|--------|      |-------------------|
                        ^
|------------|          |
| Categories | ---------|
|------------|

This way there is a single row for each Topic, a single row for each Level and a single row for each Category. The relationship logic will be contained in a new table so everything stays DRY.

5 Comments

stefano_cdn Over a year ago

That sounds good but the problem is when you request the topics in an index action, how do fetch the associated levels and categories without doing N+1 queries? Do you have to issue 3 different queries (Category.all, Topic.all and Level.all)?

yez Over a year ago

You can use references and includes: It might look something like Topic.where(id: 1).includes(:levels).references(:levels)

stefano_cdn Over a year ago

To fetch a single Topic that's the way, but it is not clear not how to proceed with the Topic index method (Topic.all) to display also the categories and levels...

yez Over a year ago

You would just remove the where clause: Topic.includes(:levels).references(:levels)

Kristján Over a year ago

You actually don't need the references in those queries. Topic.includes will load everything it needs using either the preload or eager_load strategy. blog.arkency.com/2013/12/rails4-preloading

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