将减少的数据从一个集合填充到另一个Meteor Js

Question

I've got a bit of a design issue.

Say ive got two collections:

Colletion A stores apples ( _id , appleName )

Collection B stores apple votes ( _id , apple_id , enum(0,1) )

I would like to then return collection A with a reduce of collection B based on the apple_id to return the value of 0's to 1's as a property on the main object. ( like a score )

Example data :

collection A array

[{_id : 1, appleName : 'grannySmith'},{_id : 2, appleName : 'greenApple'},{_id : 3, appleName : 'anotherApple'}]

collection B array

[{_id : 1, appleId : 1, vote : 0}, {_id : 2, appleId : 1, vote : 1}, {_id : 3, appleId : 1, vote : 1}]

From this Im looking for collection A to return with a reduce of B

collection A array with reduce

[{_id : 1, appleName : 'grannySmith', score : 2},{_id : 2, appleName : 'greenApple'},{_id : 3, appleName : 'anotherApple'}]

See as it now has a score of 2 from the reduce of collection B with linked IDs

Answer 1

You would use aggregation to achieve it:

db.apple.aggregate([
  {
    $lookup: {
      from: 'score',
      localField: '_id',
      foreignField: 'appleId',
      as: 'scores',
    },
  }, {
    $unwind: {
      path: '$scores',
      preserveNullAndEmptyArrays: true,
    },
  }, {
    $group: {
      _id: '$_id',
      appleName: {
        $first: '$appleName',
      },
      score: {
        $sum: '$scores.vote',
      }
    },
  }
])

Note: I use $lookup in the first stage of this aggregation, you need to use Mongo 3.2 or above in order for it to work.