CODEWITHSUNDEEP
mongodb
jinose c
jinose c

Reputation: 63

How can I combine more than two collections in Mongo

I have more than two collections. Each collection has fields which are similar to the foreign key in MySQL. How can I display the whole collection's data. my example colletions.

collection one:

var mongoose = require('mongoose');

module.exports = mongoose.model('Addmodule',{

    chapterid: {type: mongoose.Schema.ObjectId, ref: 'Addchapter' },
    modul: String
});

collection two:

var mongoose = require('mongoose');

module.exports = mongoose.model('Addchapter',{

    subjectid: {type: mongoose.Schema.ObjectId, ref: 'Addsubject' },
    chapter: String
});

collection three:

var mongoose = require('mongoose');

module.exports = mongoose.model('Addsubject',{

    classid: {type: mongoose.Schema.ObjectId, ref: 'Addclass' },
    subject: String
});

Upvotes: 0

Views: 808

Answers (2)

Semih Gokceoglu
Semih Gokceoglu

Reputation: 1428

With new version of mongodb(3.2), you can merge the collections. Please take a look $lookup aggregation method. Also you can check the examples in this page. If you have further questions, please feel free to ask, because your question is too general.

{
   $lookup:
     {
       from: <collection to join>,
       localField: <field from the input documents>,
       foreignField: <field from the documents of the "from" collection>,
       as: <output array field>
     }
}

Edit 1:

You do not need to use '$lookup' method in this case. You can simply use populate method. Like:

Addmodule
  .find({})
  .populate({
    path: 'chapterid',
    populate: {
      path: 'subjectid',
      populate: { path: 'classid', model: 'Addclass' }
    }
  })
  .exec(function(err, data) {
    if (err) return handleError(err);
    res.json(data);
  });

or simply

Addmodule
  .find({})
  .populate({
        path: 'chapterid.subjectid.classid',
        model: 'Addclass'           
  })
  .exec(function(err, data) {
    if (err) return handleError(err);
    res.json(data);
  });

Because I would not suggest to use lookup if you do not merge 2 array collection (like left join in sql). I can try to add an example if you would like to see lookup version.

Upvotes: 2

Rubin Porwal
Rubin Porwal

Reputation: 3845

MongoDB doesn't support joins. But from version 3.2 they have added support of left outer join through $lookup operator in aggregation pipeline.

For more detailed description regarding $lookup operator please refer the documentation mentioned in following url

https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/

Upvotes: 0

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