rails 5 左外连接

Question

Need Clarification about left outer joins. I have a model named CLub

class Club < ApplicationRecord
  has_many :club_members, dependent: :destroy
end

another model named ClubMember

class ClubMember < ApplicationRecord
  belongs_to :club
end

When I run a query like

Club.left_outer_joins(:club_members)

it gives me all record from lett table

But with something like

Club.left_outer_joins(:club_members).where('club_members.is_assigned =?',true)

Its returning only those entries which exist in ClubMember, I was expecting it to return entries from left table which don't exist in ClubMember & existing entries in ClubMember with status TRUE too. Just leave those entries which exist in ClubMemebr with status FALSE.

I have a wrong concept or doing something wrong?

Answer 1

LEFT JOIN:

The LEFT JOIN returns all rows from the left table, even if there are no matches in the right table.

This means that if the ON clause matches 0 (zero) records in the right table; the join will still return a row in the result, but with NULL in each column from the right table.

So if we apply the condition on any column in right table. Then it will returns the results according to the condition. If we want the records where there is notmatch. Than we should add a condition where right table has null values also. That's what i have added in the code.

Club.left_outer_joins(:club_members)
    .where("club_members.is_assigned =true OR club_members.is_assigned is null")

Answer 2

For return the left side plus the coincidences you could add an OR . something like this:

Club.left_outer_joins(:club_members).where('club_members.is_assigned =?',true).or(where('condition with the left side'))