Re: [Rails] Select and inner join

On 15 February 2013 10:05, Lorenz Blackbird <lists@ruby-forum.com> wrote:
> I have a many-to-many relationship in my course model in this way:
>
> has_many :course_paragraphs
> has_many :paragraphs, :through => :course_paragraphs
>
> I use an inner join to select the paragraphs of my course:
>
> Paragraph.joins(:course_paragraphs => :course)
> .where('course_paragraphs.course_id' => @course.id)

You don't need to do that. If you have a course in @course then its
paragraphs are just @course.paragraphs. It is rare to have to use
joins when working with Rails if you have got the associations right.

>
> This works, but I want to select an array containing only the ids of
> paragraphs.

I think this should work.
@course.paragraphs.select( :id )

Colin

>
> By this solution I get an error:
>
> @p_ids = Paragraph.select(:id)
> .joins(:course_paragraphs => :course)
> .where('course_paragraphs.course_id' => @course.id)
>
> Mysql2::Error: Column 'id' in field list is ambiguous: SELECT id FROM
> `paragraphs` INNER JOIN `course_paragraphs` ON
> `course_paragraphs`.`paragraph_id` = `paragraphs`.`id` INNER JOIN
> `courses` ON `courses`.`id` = `course_paragraphs`.`course_id` WHERE
> `course_paragraphs`.`course_id` = 1
>
> By this solution, the array contain hexadecimal values:
>
> @p_ids = Paragraph.select('paragrahs.id')
> .joins(:course_paragraphs => :course)
> .where('course_paragraphs.course_id' => @course.id)
>
> Where in my problem?
>
> Thanks to all.
>
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