select
select(value = Proc.new)Works in two unique ways.
First: takes a block so it can be used just like Array#select.
Model.scoped.select { |m| m.field == value }
This will build an array of objects from the database for the scope, converting them into an array and iterating through them using Array#select.
Second: Modifies the SELECT statement for the query so that only certain fields are retrieved:
>> Model.select(:field) => [#<Model field:value>]
Although in the above example it looks as though this method returns an array, it actually returns a relation object and can have other query methods appended to it, such as the other methods in ActiveRecord::QueryMethods.
The argument to the method can also be an array of fields.
>> Model.select([:field, :other_field, :and_one_more]) => [#<Model field: "value", other_field: "value", and_one_more: "value">]
Any attributes that do not have fields retrieved by a select will raise a ActiveModel::MissingAttributeError when the getter method for that attribute is used:
>> Model.select(:field).first.other_field => ActiveModel::MissingAttributeError: missing attribute: other_field
3Notes
Arguments for .select must be array
Model.select(:field, :other_field, :and_one_more) has a typo. It must take an array of arguments as the description states:
Model.select([:field, :other_field, :and_one_more])
arguments do not need to be an array
it's a small point, but if you look at the source, the method is defined with the splat operator in the arguments: def select (*fields) this means that a list of arguments is automatically converted to an array. There is no typo in the description above.
It will also work to pass an array: select([:field1, :field2]) although the select method interprets this as a single argument, and places it into an array (due to the splat operator), this is then passed to the _select(*fields) method, which immediately calls fields.flatten!
So either a list or an array may be passed, both will work.
Counting with select
If you try to write
Model.select('field_one', 'field_two AS something').count
it will fail (at least for Rails 5.0) with the message PG::SyntaxError: ERROR: syntax error at or near "AS". In order to fix that issue, you should write
Model.select('field_one', 'field_two AS something').count(:all)
