Have you heard of the #pluck method from Rails’ Active Record? Read on to see what it does and how it can be used to easily query single columns from your database!

The use case

Sometimes when retrieving data from the database, we are only interested in certain attributes (e.g., just emails of all the users, instead of data residing in all columns of the users table).

The naïve approach would be to load all the data into the memory and then map it so that only the subset of the data is left, e.g.:

# SELECT "users".* FROM "users"

#=> ["kate@example.com", "john@example.org", "janet@example.com"]

This is quite inefficient.

First of all, the SQL query is retrieving data from all columns from the table. Secondly, the ActiveRecrodCollection object is created in the memory. Its members are initialized with all the data retrieved from the DB. The map at the end returns a regular Ruby Array object, making this intermediate step unnecessary.

Knowing we only really need to load the data from the email column, we could improve the above by chaining the Active Record’s #select method:

# SELECT "users"."email" FROM "users"

#=> ["kate@example.com", "john@example.org", "janet@example.com"]

The SQL query is limited to one column now, but the ActiveRecord objects are still being created in the intermediate step before map. Granted, they contain less data now (as the attributes we didn’t select are omitted), but it’s still quite unnecessary in this use case.

Using the #pluck

The good news is, we can use #pluck instead to solve this issue. It’s as simple as:

# SELECT "users"."email" FROM "users"

#=> ["kate@example.com", "john@example.org", "janet@example.com"]

The SQL is still retrieving just the data we need. The data is then returned in the Plain Old Ruby Array, and no ActiveRecord object is being created in between!

Retrieving data from multiple columns

The #pluck method works with multiple DB columns, too. Just pass their names as arguments.

Instead of this:

User.select(:id, :email).map { |user| [user.id, user.email] }
# SELECT "users"."id", "users"."email" FROM "users"

#=> [[1, "kate@example.com"], [4, "john@example.org"], [5, "janet@example.com"]]

Just do this:

User.pluck(:id, :email)
# SELECT "users"."id", "users"."email" FROM "users"

#=> [[1, "kate@example.com"], [4, "john@example.org"], [5, "janet@example.com"]]


When querying the database, it’s important to be mindful about what data we retrieve and what is being done with it. The #pluck method both keeps the SQL query to the minimum and avoids the ActiveRecord object creation overhead when it’s not needed in a given scenario.

Have you used the #pluck method in your project? Please share in the comments.

And if your project could benefit from faster CI builds, consider using Knapsack Pro to achieve that!