Activerecords Select(:Id).Collect VS. Pluck(:Id) Methods: Why Is Pure Ar "Pluck" Slower

ActiveRecords select(:id).collect vs. pluck(:id) methods: Why is pure AR pluck slower?

Your benchmark is inaccurate. First of all, as you can see, both executions on the database side triggers the same query

SELECT "articles"."id" FROM "articles"

Therefore, the database time should be considered irrelevant. Clearly the two queries had different execution time as shown by the console, but this is normal as if you run the same query 100 times the execution time can be different each time as it depends by a variety of variables such as the machine load, the database state, etc.

Since the database execution time can be considered equivalent, it's irrelevant for the benchmark.

Therefore, what you need to compare is the Ruby execution time and allocation. Pluck is supposed to be faster and more lightweight as compared to collect it doesn't allocate ActiveRecord objects, rather it returns only the selected values.

If you really want to benchmark the methods, you should mock the database time (which is clearly variable but irrelevant for this benchmark) and only benchmark allocation and the two different Ruby methods.

Long story short, pluck is generally more efficient.

Rails where condition using NOT NIL

Rails 4+

ActiveRecord 4.0 and above adds where.not so you can do this:

Foo.includes(:bar).where.not('bars.id' => nil)
Foo.includes(:bar).where.not(bars: { id: nil })

When working with scopes between tables, I prefer to leverage merge so that I can use existing scopes more easily.

Foo.includes(:bar).merge(Bar.where.not(id: nil))

Also, since includes does not always choose a join strategy, you should use references here as well, otherwise you may end up with invalid SQL.

Foo.includes(:bar)
   .references(:bar)
   .merge(Bar.where.not(id: nil))

Rails 3

The canonical way to do this with Rails 3:

Foo.includes(:bar).where("bars.id IS NOT NULL")

Find model records by ID in the order the array of IDs were given

Note on this code:

ids.each do |i|
  person = people.where('id = ?', i)

There are two issues with it:

First, the #each method returns the array it iterated on, so you'd just get the ids back. What you want is a collect

Second, the where will return an Arel::Relation object, which in the end will evaluate as an array. So you'd end up with an array of arrays. You could fix two ways.

The first way would be by flattening:

ids.collect {|i| Person.where('id => ?', i) }.flatten

Even better version:

ids.collect {|i| Person.where(:id => i) }.flatten

A second way would by to simply do a find:

ids.collect {|i| Person.find(i) }

That's nice and simple

You'll find, however, that these all do a query for each iteration, so not very efficient.

I like Sergio's solution, but here's another I would have suggested:

people_by_id = Person.find(ids).index_by(&:id) # Gives you a hash indexed by ID
ids.collect {|id| people_by_id[id] }

I swear that I remember that ActiveRecord used to do this ID ordering for us. Maybe it went away with Arel ;)