The other day, while working on a rails application I wrote the following line of code: Poem.where(...).order("created_at DESC").limit(10)

Then I had a thought: If the where method returns a large collection of poems, will I be loading hundreds of poems in memory when I only need a hundred?

A quick check on my rails console showed me that this code executes one database query: SELECT "poems".* FROM "poems" WHERE (...) ORDER BY created_at DESC LIMIT ? [["LIMIT", 10]]

Somehow, the where, order, and limit methods combine into one database query instead of executing one after the other. In the past, I’ve taken this functionality for granted without considering how the database queries are executed. However, I decided to finally dig deeper and understand what happens under the hood.

Active Record Relations

Methods such as where, order, and limit wait to query the database until the result is required by the application. It might appear as thought the methods return an array of objects. However, if you run the query in the rails console, the result is an ActiveRecord::Relation not an array.

Query methods don’t execute requests directly to the database. Instead, they return a Relation object. This allows you to chain multiple query methods together without repeatedly querying the database. When you run Poem.where(...), it returns a Relation. order takes the Relation and adds its own criteria to it and returns another Relation. This Relation is used by the limit query, which also returns a Relation.

The Relation will only execute as a command to the database when you need to know what’s inside the Relation. For example, it will not execute a database query if you ask for the class of the result. This is because, the relation does not need the database information to determine the class. In comparison, when you ask for the title of the first poem, it queries the database and returns the result.

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> Poem.where(search_query).order("created_at DESC").limit(10).class
=> Poem::ActiveRecord_Relation

> Poem.where(search_query).order("created_at DESC").limit(10).first.title
  Poem Load (0.3ms)  SELECT  "poems".* FROM "poems" WHERE (...) ORDER BY created_at DESC LIMIT ?  [["LIMIT", 10]]
=> "Roses"

Do All Query Methods Return Relations?

No! Query methods that return a collection of objects such as where and group return an instance of ActiveRecord::Relation. Methods which return a single entity, such as find and first, will return an instance of the model.

Therefore, it’s good to keep in mind which methods return Relations and which return model objects. As well, you need to remember to put query methods that return a single instance at the end of your query since they execute immediately.

More Resources

For more information about the internals of Relations and how they are created and executed, I’d highly suggest watching this Railscast Episode on ActiveRecord::Relation It’s only 11 minutes long and helped clarify my understanding of Active Record Relations