Qfyilyi

I have this collection:

// collection

[

  {_id: 1, name: 'Luigi', childs: [{name: 'one'}, {name: 'two'}], dad_id: 9]},

  {_id: 1, name: 'Mario', childs: [{name: 'four'}, {name: 'five'}], dad_id: 8]},

  {_id: 1, name: 'Alessandro', childs: [{name: 'seven'}, {name: 'six'}], dad_id: 9]},

]

and apply this filter to it

result = collection.find({ dad_id: 9 })

Then I want to aggregate the results and get all the childs singularly, I start with unwinding them
(then I 'll make a projection, etc..) but I already encounter a behavior that I do not understand:
the result contains also the documents with dad_id is 8, even if they were already excluded by my query.

result.aggregate([

  { "$unwind"=> "$childs" },

]).each do |e| ... end

// => [

  {_id: 1, name: 'Luigi', childs: {name: 'one'}, dad_id: 9]},

  {_id: 1, name: 'Luigi', childs: {name: 'two'}, dad_id: 9]},

  {_id: 1, name: 'Luigi', childs: {name: 'five'}, dad_id: 8]},

  {_id: 1, name: 'Luigi', childs: {name: 'four'}, dad_id: 8]},

  {_id: 1, name: 'Luigi', childs: {name: 'seven'}, dad_id: 9]},

  {_id: 1, name: 'Luigi', childs: {name: 'six'}, dad_id: 9]},

]

What am I missing?

You can not chain input from one query to another query like that.

Either use search query ex. Model.find(id) or aggregation framework.

Aggregation framework provides you the functionality to create a pipeline (ex. match,unwind,lookup,project).

To utilize mongodb indexing always try to use "$match" first in the pipeline

match = { "$match" => { "dad_id" =>9} }

unwind = {"$uwind"=>"$childs"}

pipeline = [match,unwind]

collection.aggregate(pipeline).each do |obj|

end