What is the difference between pluck and collect in Rails?

Here are two sample codes.

First one with collect:

User.first.gifts.collect(&:id)

Second one with pluck:

User.first.gifts.pluck(:id)

Is there any difference between them in performance or something else?

pluck is on the db level. It will only query the particular field. See this.

When you do:

 User.first.gifts.collect(&:id)

You have objects with all fields loaded and you simply get the id thanks to the method based on Enumerable.

So:

• if you only need the id with Rails 4, use ids: User.first.gifts.ids

• if you only need some fields with Rails 4, use pluck: User.first.gifts.pluck(:id, :name, ...)

• if you only need one field with Rails 3, use pluck: User.first.gifts.pluck(:id)

• if you need all fields, use collect

• if you need some fields with Rails 4, still use pluck

• if you need some fields with Rails 3, use selectand collect

Yes. According to Rails guides, pluck directly converts a database result into an array, without constructing ActiveRecord objects. This means better performance for a large or often-running query.

In addition to @apneadiving's answer, pluck can take both single and multiple column names as argument:

Client.pluck(:id, :name)
# SELECT clients.id, clients.name FROM clients
# => [[1, 'David'], [2, 'Jeremy'], [3, 'Jose']]

The basic and main difference is that Pluck applies on db level and collect get all data and then return record to you when you need all records use collect and when few fields then use pluck

If there is a case where you are using few attributes of the retrieved record. In such cases you should use pluck .

User.collect(&:email)

In above example if you only need email attribute than you are wasting memory and time. Because it will retrieve all the columns from user table in the database, allocates the memory for each attributes (including the attributes which you will never use)

NOTE: pluck does not return ActiveRecord_Relation of the user

Late to the party, but if you would like a deeper pluck you can use includes:

User.all.includes(:groups).pluck("users.id", "users.name", "groups.id", "groups.name")

This results in a bigger array like

[
  [12, "Sam", "FriendsRUs", 4],
  ...
]