问题
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?
回答1:
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
idwith Rails 4, useids:User.first.gifts.idsif 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
collectif you need some fields with Rails 4, still use
pluckif you need some fields with Rails 3, use
selectandcollect
回答2:
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']]
回答3:
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
回答4:
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
来源：https://stackoverflow.com/questions/12176102/what-is-the-difference-between-pluck-and-collect-in-rails