How to get a single column's values into an array

Question

Right now I'm doing something like this to select a single column of data:

points = Post.find_by_sql("select point from posts")

Then passing them to a method, I'd like my method to remain agnostic, and now have to call hash.point from within my method. How can I quickly convert this into an array and pass the data set to my method, or is there a better way?

score 198 · Accepted Answer · edited Mar 23 '18 at 16:19

198

In Rails 3.2 there is a pluck method for this

Just like this:

Person.pluck(:id) # SELECT people.id FROM people
Person.pluck(:role).uniq # unique roles from array of people
Person.distinct.pluck(:role) # SELECT DISTINCT role FROM people SQL
Person.where(:confirmed => true).limit(5).pluck(:id)

Difference between uniq and distinct

edited Mar 23 '18 at 16:19

BDC

63
1
9

answered Mar 26 '12 at 13:09

alony

10,725
3
39
46

How can an array inherit a query result's ordering with pluck? – franklin stine Mar 26 '12 at 14:29
3

Post.order(:score).pluck(:score) is the solution. Thanks. – franklin stine Mar 26 '12 at 16:54
6

To pluck ids there is a special method: `Person.ids`. – shock_one Mar 24 '14 at 15:23
Also look into Ernie Miller's gem Valium https://github.com/ernie/valium especially if you're on an older Rails or want multiple columns – James Daniels Oct 18 '14 at 21:14
As of Rails 5.0, it is now recommended to use `distinct` instead of `uniq`: so `Person.distinct.pluck(:role)` – David Aug 09 '16 at 13:38

Patrick Oscity · Answer 2 · 2014-03-05T23:16:36.880

17

You should use the pluck method as @alony suggested. If you are stuck before Rails 3.2 you can use the ActiveRecord select method together with Array#map:

Post.select(:point).map(&:point)
#=> ["foo", "bar", "baz"]

before Ruby 1.9 you'd have to do .map{|x| x.title} though, because Symbol#to_proc (aliased by the unary & operator) is not defined in earlier versions of Ruby.

edited Mar 05 '14 at 23:16

answered Mar 26 '12 at 10:02

Patrick Oscity

53,604
17
144
168

Thanks. I saw you commented on the new Pluck method. Can you do the same with that? – franklin stine Mar 26 '12 at 16:45
Yes, if you read @alony's answer carefully, you will notice that she already included an example that does this. – Patrick Oscity Mar 26 '12 at 18:01

score 5 · Answer 3 · answered Mar 26 '12 at 10:26

5

If you see the definition of select_values , then it using 'map(&:field_name)'

  def select_values(arel, name = nil)
    result = select_rows(to_sql(arel), name)
    result.map { |v| v[0] }
  end

The common and general Rails way to collect all the fields values in array is like :

points = Post.all(:select => 'point').map(&:point)

answered Mar 26 '12 at 10:26

Vik

5,931
3
31
38

Good point. Thank you. This works great, and lets me still apply order and other things to the query, unlike the Post.select example. – franklin stine Mar 26 '12 at 10:47
I dont agree with you, @franklinstine: `Post.select(:point).limit(1)` performs `SELECT point FROM "posts" LIMIT 1` whereas `Post.all(:select => :point).limit(1)` raises a `NoMethodError`. Please correct me if i'm wrong. – Patrick Oscity Mar 26 '12 at 12:53
I agree with your comments. But I am not getting where mention in the post to use : Post.all(:select => :point).limit(1) Ofcourse it will give error "undefined method `limit'" – Vik Mar 26 '12 at 13:15
@franklinstine said he prefers your method because he could chain other statements after it, i just wanted to make clear that this is not true – Patrick Oscity Mar 26 '12 at 13:57
Yes @padde you mis-inferred. I was saying you can still apply order and other query statements such as: Post.all(:select => 'score', :order => 'score ASC').map(&:score). – franklin stine Mar 26 '12 at 14:13
Ok, now i see. I have to admit though, that using the `pluck` method as @alony suggested is the most elegant way to do this. – Patrick Oscity Mar 26 '12 at 14:19

score 3 · Answer 4 · answered Mar 26 '12 at 09:58

3

points = Post.all.collect {|p| p.point}

answered Mar 26 '12 at 09:58

rajibchowdhury

5,264
2
14
7

2

This works, but you select all fields from the database and then filter the result in Ruby whereas the `select` statement only fetches the specified columns. – Patrick Oscity Mar 26 '12 at 10:03

How to get a single column's values into an array

4 Answers4

Linked