Mongodb, aggregate query with $lookup

by

For any particular person document, you can use the populate() function like 

var query = mongoose.model("person").find({ "name": "foo" }).populate("projects.tags");

And if you want to search for any persons that have any tag with ‘MongoDB’ or ‘Node JS’ for example, you can include the query option in the populate() function overload as:

var query = mongoose.model("person").find({ "name": "foo" }).populate({
    "path": "projects.tags",
    "match": { "en": { "$in": ["MongoDB", "Node JS"] } }
});

If you want all tags existing in "project.tags" for all persons, then aggregation framework is the way to go. Consider running this pipeline on the person collection and uses the $lookup operator to do a left join on the tags collection:

mongoose.model('person').aggregate([
    { "$unwind": "$projects" },
    { "$unwind": "$projects.tags" },
    {
        "$lookup": {
            "from": "tags",
            "localField": "projects.tags",
            "foreignField": "_id",
            "as": "resultingTagsArray"
        }
    },
    { "$unwind": "$resultingTagsArray" },
    {
        "$group": {
            "_id": null,
            "allTags": { "$addToSet": "$resultingTagsArray" },
            "count": { "$sum": 1 }
        }
    }
 ]).exec(function(err, results){
    console.log(results);
 })

For any particular person then apply a $match pipeline as the first step to filter the documents: 

mongoose.model('person').aggregate([
    { "$match": { "name": "foo" } },
    { "$unwind": "$projects" },
    { "$unwind": "$projects.tags" },
    {
        "$lookup": {
            "from": "tags",
            "localField": "projects.tags",
            "foreignField": "_id",
            "as": "resultingTagsArray"
        }
    },
    { "$unwind": "$resultingTagsArray" },
    {
        "$group": {
            "_id": null,
            "allTags": { "$addToSet": "$resultingTagsArray" },
            "count": { "$sum": 1 }
        }
    }
 ]).exec(function(err, results){
    console.log(results);
 })

Another workaround if you are using MongoDB versions >= 2.6 or <= 3.0 which do not have support for the $lookup operator is to populate the results from the aggregation as:

mongoose.model('person').aggregate([
    { "$unwind": "$projects" },
    { "$unwind": "$projects.tags" },    
    {
        "$group": {
            "_id": null,
            "allTags": { "$addToSet": "$projects.tags" }
        }
    }
 ], function(err, result) {
    mongoose.model('person')
    .populate(result, { "path": "allTags" }, function(err, results) {
        if (err) throw err;
        console.log(JSON.stringify(results, undefined, 4 ));
    });
});

More Related Contents:

  1. Does MongoDB’s $in clause guarantee order
  2. Moongoose aggregate $match does not match id’s
  3. How to return just the nested documents of an array from all documents
  4. MongoDB: Calculate dwell time between every status value change
  5. MongoDB: how to parse date in 3.6 mongoDb version?
  6. Mongodb aggregation $group, restrict length of array
  7. $project in $lookup mongodb
  8. Group result by 15 minutes time interval in MongoDb
  9. Find in Double Nested Array MongoDB
  10. Check if every element in array matches condition
  11. MongoDB nested lookup with 3 levels
  12. Get a count of total documents with MongoDB when using limit
  13. Include all existing fields and add new fields to document
  14. Matching ObjectId to String for $graphLookup
  15. mongodb count num of distinct values per field/key
  16. MongoDB aggregation comparison: group(), $group and MapReduce
  17. Mongodb aggregation lookup with conditions
  18. MongoDB group by array inner-elements
  19. Exception: can’t convert from BSON type EOO to Date
  20. Multiple Counts with single query in mongodb
  21. How to return array of string with mongodb aggregation
  22. MongoDB lookup when foreign field is an array of objects
  23. Get n-th element of an array in MongoDB
  24. Geospatial $near within current document field value
  25. How do I rename fields when performing search/projection in MongoDB?
  26. mongodb aggregation framework match by nested documents
  27. Find documents whose array field contains at least n elements of a given array
  28. count array occurrences across all documents with mongo
  29. mongodb aggregate query isn’t returning proper sum on using $sum
  30. Return the last “true” value for each group

Leave a Comment