Rails: select unique values from a column

I already have a working solution, but I would really like to know why this doesn't work:

ratings = Model.select(:rating).uniq
ratings.each { |r| puts r.rating }

It selects, but don't print unique values, it prints all values, including the duplicates. And it's in the documentation: http://guides.rubyonrails.org/active_record_querying.html#selecting-specific-fields

Model.select(:rating)

The result of this is a collection of Model objects. Not plain ratings. And from uniq's point of view, they are completely different. You can use this:

Model.select(:rating).map(&:rating).uniq

or this (most efficient):

Model.uniq.pluck(:rating)

Rails 5+

Model.distinct.pluck(:rating)

Update

Apparently, as of rails 5.0.0.1, it works only on "top level" queries, like above. Doesn't work on collection proxies ("has_many" relations, for example).

Address.distinct.pluck(:city) # => ['Moscow']
user.addresses.distinct.pluck(:city) # => ['Moscow', 'Moscow', 'Moscow']

In this case, deduplicate after the query

user.addresses.pluck(:city).uniq # => ['Moscow']

If you're going to use Model.select, then you might as well just use DISTINCT, as it will return only the unique values. This is better because it means it returns less rows and should be slightly faster than returning a number of rows and then telling Rails to pick the unique values.

Model.select('DISTINCT rating')

Of course, this is provided your database understands the DISTINCT keyword, and most should.

This works too.

Model.pluck("DISTINCT rating")

If you want to also select extra fields:

Model.select('DISTINCT ON (models.ratings) models.ratings, models.id').map { |m| [m.id, m.ratings] }

Model.uniq.pluck(:rating)

# SELECT DISTINCT "models"."rating" FROM "models"

This has the advantages of not using sql strings and not instantiating models

Model.select(:rating).uniq

This code works as 'DISTINCT' (not as Array#uniq) since rails 3.2

Model.select(:rating).distinct

Another way to collect uniq columns with sql:

Model.group(:rating).pluck(:rating)

If I am going right to way then :

Current query

Model.select(:rating)

is returning array of object and you have written query

Model.select(:rating).uniq

uniq is applied on array of object and each object have unique id. uniq is performing its job correctly because each object in array is uniq.

There are many way to select distinct rating :

Model.select('distinct rating').map(&:rating)

or

Model.select('distinct rating').collect(&:rating)

or

Model.select(:rating).map(&:rating).uniq

or

Model.select(:name).collect(&:rating).uniq

One more thing, first and second query : find distinct data by SQL query.

These queries will considered "london" and "london   " same means it will neglect to space, that's why it will select 'london' one time in your query result.

Third and forth query:

find data by SQL query and for distinct data applied ruby uniq mehtod. these queries will considered "london" and "london " different, that's why it will select 'london' and 'london ' both in your query result.

please prefer to attached image for more understanding and have a look on "Toured / Awaiting RFP".

If anyone is looking for the same with Mongoid, that is

Model.distinct(:rating)

Some answers don't take into account the OP wants a array of values

Other answers don't work well if your Model has thousands of records

That said, I think a good answer is:

    Model.uniq.select(:ratings).map(&:ratings)
    => "SELECT DISTINCT ratings FROM `models` " 

Because, first you generate a array of Model (with diminished size because of the select), then you extract the only attribute those selected models have (ratings)

You can use the following Gem: active_record_distinct_on

Model.distinct_on(:rating)

Yields the following query:

SELECT DISTINCT ON ( "models"."rating" ) "models".* FROM "models"

In my scenario, I wanted a list of distinct names after ordering them by their creation date, applying offset and limit. Basically a combination of ORDER BY, DISTINCT ON

All you need to do is put DISTINCT ON inside the pluck method, like follow

Model.order("name, created_at DESC").offset(0).limit(10).pluck("DISTINCT ON (name) name")

This would return back an array of distinct names.

Model.pluck("DISTINCT column_name")