CODEWITHSUNDEEP
ruby-on-railsruby-on-rails-3activerecordhas-manyrelation
Undistraction
Undistraction

Reputation: 43401

Unique Associations From ActiveRecordRelation

I have a Project model that has-many People.

Given an ActiveRecordRelation containing a large number of Projects, how can I efficiently get an ActiveRecordRelation containing all unique People associated with all the projects?

Basically I'm looking for an efficient way of doing:

@people = @projects.flat_map {|project| project.people}.uniq

Upvotes: 0

Views: 59

Answers (1)

user419017
user419017

Reputation:

Eager loading does not appear to work correctly with has_many, or is not clever enough, at least. Rails will eager load the association, but then resort to a query when you call that relation on a specific instance.

Example (not actual queries):

@articles = Article.includes(:comments)
#> Article Load: SELECT * from articles
#> Comment Load: SELECT * from comments where article_id IN(1,2,3,4)

@articles.first.comments
#> Comment Load: SELECT * from comments where article_id IN(1)

In the @articles.first.comments, I would expect to see Comment Load (Cache) or something similar, as that comment has already been loaded.

There might be a better alternative, but I would simply perform a custom query such as:

@people = People.where("project_id IN(?)", @projects.map(&:id))

Rails 4:

@people = People.where(project_id: @projects.map(&:id))

Upvotes: 1

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