Rails 活动记录查询与“存在”的关联

Question

I am working on an app that allows Members to take a survey (Member has a one to many relationship with Response). Response holds the member_id, question_id, and their answer.

The survey is submitted all or nothing, so if there are any records in the Response table for that Member they have completed the survey.

My question is, how do I re-write the query below so that it actually works? In SQL this would be a prime candidate for the EXISTS keyword.

 def surveys_completed
    members.where(responses: !nil ).count
 end 

Answer 1

You can use includes and then test if the related response(s) exists like this:

def surveys_completed
  members.includes(:responses).where('responses.id IS NOT NULL')
end

Here is an alternative, with joins :

def surveys_completed
  members.joins(:responses)
end

The solution using Rails 4 :

def surveys_completed
  members.includes(:responses).where.not(responses: { id: nil })
end

---

Alternative solution using activerecord_where_assoc : This gem does exactly what is asked here: use EXISTS to to do a condition. It works with Rails 4.1 to the most recent.

members.where_assoc_exists(:responses)

It can also do much more!

---

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Answer 2

You can use SQL EXISTS keyword in elegant Rails-ish manner using Where Exists gem:

members.where_exists(:responses).count

Of course you can use raw SQL as well:

members.where("EXISTS" \
  "(SELECT 1 FROM responses WHERE responses.member_id = members.id)").
  count

Answer 3

You can also use a subquery:

members.where(id: Response.select(:member_id))

In comparison to something with includes it will not load the associated models (which is a performance benefit if you do not need them).

Answer 4

If you are on Rails 5 and above you should use left_joins . Otherwise a manual "LEFT OUTER JOINS" will also work. This is more performant than using includes mentioned in https://stackoverflow.com/a/18234998/3788753 . includes will attempt to load the related objects into memory, whereas left_joins will build a "LEFT OUTER JOINS" query.

def surveys_completed
  members.left_joins(:responses).where.not(responses: { id: nil })
end

Even if there are no related records (like the query above where you are finding by nil) includes still uses more memory. In my testing I found includes uses ~33x more memory on Rails 5.2.1. On Rails 4.2.x it was ~44x more memory compared to doing the joins manually.

See this gist for the test: https://gist.github.com/johnathanludwig/96fc33fc135ee558e0f09fb23a8cf3f1

Answer 5

where.missing (Rails 6.1+)

Rails 6.1 introduces a new way to check for the absence of an association - where.missing .

Please, have a look at the following code snippet:

# Before:
Post.left_joins(:author).where(authors: { id: nil })

# After:
Post.where.missing(:author)

And this is an example of SQL query that is used under the hood:

Post.where.missing(:author)
# SELECT "posts".* FROM "posts"
# LEFT OUTER JOIN "authors" ON "authors"."id" = "posts"."author_id"
# WHERE "authors"."id" IS NULL

As a result, your particular case can be rewritten as follows:

def surveys_completed
  members.where.missing(:response).count
end 

Thanks.

Sources:

• where.missing official docs .

• Pull request .

• Article from the Saeloun blog .

Notes:

• where.associated - a counterpart for checking for the presence of an association is also available starting from Rails 7.
• See offical docs and this answer .