Rails从两个表中的关联加入

Question

I have to tables: Associates and Departments

One associate belongs to a department (belongs_to) and one department has many associates (has_many)

What I', trying to do is count number of associates on each department and make a pie chart with the data, i got the chart working with something basic (Department.count)

I just started with rails and I'm having issues with the query

Associate.joins(:department).group(:name).count

what i get is the following

Mysql2::Error: Column 'name' in field list is ambiguous: SELECT COUNT(*) AS count_all, name AS name FROM associates INNER JOIN departments ON departments . id = associates . department_id GROUP BY name

I tried doing a select, find and I just cant seem to get it right

Answer 1

If you have your associations setup correctly, I believe you should be able to do something like this:

Department.find(<some_id>).associates.count

Or if you have all the departments, and you're looping through them, let's say in a view and you want to get all the counts, you can do something like this:

@departments = Deparment.all

@departments.each do |department|
  department.associates.count
end

You're associate table will need a department_id column on it.

Answer 2

As stated in the comment on your post, you have to resolve the ambiguous name attributed by specifying the table. You can do so either with the string as suggested, or by passing a block to group_by :

Associate.joins(:department).group_by(&:name).count