CODEWITHSUNDEEP
ruby-on-railsmongoid
user27111987
user27111987

Reputation: 1085

Rails - Mongoid querying on a association field

I have models like this

class TestOne
  field value

  belongs_to test_two ,:class_name => 'TestTwo'
end

class TestTwo
  field name
  field start_time, :type => DateTime

  has_many TestOne

end

I am running a query of range like this

result = TestOne.where(:value => "some_value" , 'test_two.start_time' => (Time.now-1.days..Time.now + 1.days).last

The result set of above query is empty even though there exists several record in TestOne that fulfills the criteria. Can any one suggest what I might be doing wrong.

Upvotes: 1

Views: 2329

Answers (1)

mu is too short
mu is too short

Reputation: 434695

A MongoDB query can only access one collection at a time, there are no joins here.

When you say:

'test_two.start_time' => (...)

MongoDB will be looking for a field in test_ones named test_two that is a hash (or array of hashes) with a start_time field inside that hash. You don't have that structure so your query doesn't find anything.

Also, you can query any MongoDB collection for any fields you like and MongoDB won't complain; documents within a collection have no set per-collection structure: any document in any collection can contain any fields with any types. That's why you can use this query without anyone complaining.

You need to do the query in two steps (i.e. do the join by hand):

test_two_ids = TestTwo.where(:start_time => (Time.now-1.days..Time.now + 1.days)).pluck(:id)
result = TestOne.where(:value => "some_value" , :test_two_id.in => test_two_ids).last

There are a couple alternatives that are more work:

  1. Keep a copy of db.test_twos.start_time in the db.test_ones collection (i.e. precompute the JOIN by denormalizing). This would require you to update the copies every time db.test_twos.start_time changes and you'd have to periodically sanity check all the copies and fix the broken ones because they will become out of sync.
  2. If you don't need TestTwo to exist on its own then embed TestTwo inside TestOne. This would leave you with a hash field called test_two inside your db.test_ones collection and your original query would work. However, you wouldn't be able to access TestTwo on its own anymore, you'd have to go through TestOne.

Upvotes: 3

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