Rails: select unique values from a column

Question

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.rat         


        

Answer 1

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"

Answer 2

Another way to collect uniq columns with sql:

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

Answer 3

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

Model.distinct(:rating)

Answer 4

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']

Answer 5

Model.select(:rating).uniq

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

Answer 6

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".