Mongoose Join List

We can use $lookup to merge the documents in two different collections of same database and it performs a left outer join on the collections.

Let us the below documents in content collection

{        "_id" : ObjectId("59ef51f106b0505f997f84c8"),        "title" : "myfavoritesong",        "description" : "A wonderful composition using string instruments"}{        "_id" : ObjectId("59ef52ad06b0505f997f84ca"),        "title" : "myfavoritestory",        "description" : "An interesting short story with a twisted ending"}

Documents in viewed collection

{        "_id" : ObjectId("59ef523706b0505f997f84c9"),        "contentid" : ObjectId("59ef51f106b0505f997f84c8"),        "viewedby" : "user1"}{        "_id" : ObjectId("59ef52f406b0505f997f84cb"),        "contentid" : ObjectId("59ef52ad06b0505f997f84ca"),        "viewedby" : "user2"}{        "_id" : ObjectId("59ef53c706b0505f997f84cc"),        "contentid" : ObjectId("59ef52ad06b0505f997f84ca"),        "viewedby" : "user3"}

Final aggregate query using $lookup by combining the two collections is

db.viewed.aggregate({    $lookup:{        from : "content",         localField: "contentid",         foreignField:"_id",        as:"viewed_contents"    }})

Result of the aggregate query for our sample data is

{        "_id" : ObjectId("59ef523706b0505f997f84c9"),        "contentid" : ObjectId("59ef51f106b0505f997f84c8"),        "viewedby" : "user1",        "viewed_contents" : [                {                        "_id" : ObjectId("59ef51f106b0505f997f84c8"),                        "title" : "myfavoritesong",                        "description" : "A wonderful composition using string instruments"                }        ]}{        "_id" : ObjectId("59ef52f406b0505f997f84cb"),        "contentid" : ObjectId("59ef52ad06b0505f997f84ca"),        "viewedby" : "user2",        "viewed_contents" : [                {                        "_id" : ObjectId("59ef52ad06b0505f997f84ca"),                        "title" : "myfavoritestory",                        "description" : "An interesting short story with a twisted ending"                }        ]}{        "_id" : ObjectId("59ef53c706b0505f997f84cc"),        "contentid" : ObjectId("59ef52ad06b0505f997f84ca"),        "viewedby" : "user3",        "viewed_contents" : [                {                        "_id" : ObjectId("59ef52ad06b0505f997f84ca"),                        "title" : "myfavoritestory",                        "description" : "An interesting short story with a twisted ending"                }        ]}

Please note you can also swap the collections from viewed as foreign and content as local

db.content.aggregate({   $lookup:{     from : "viewed",      localField: "_id",      foreignField:"contentid",     as:"contents_viewed_by"   }})

Result of this aggregate query is as follows

{        "_id" : ObjectId("59ef51f106b0505f997f84c8"),        "title" : "myfavoritesong",        "description" : "A wonderful composition using string instruments",        "contents_viewed_by" : [                {                        "_id" : ObjectId("59ef523706b0505f997f84c9"),                        "contentid" : ObjectId("59ef51f106b0505f997f84c8"),                        "viewedby" : "user1"                }        ]}{        "_id" : ObjectId("59ef52ad06b0505f997f84ca"),        "title" : "myfavoritestory",        "description" : "An interesting short story with a twisted ending",        "contents_viewed_by" : [                {                        "_id" : ObjectId("59ef52f406b0505f997f84cb"),                        "contentid" : ObjectId("59ef52ad06b0505f997f84ca"),                        "viewedby" : "user2"                },                {                        "_id" : ObjectId("59ef53c706b0505f997f84cc"),                        "contentid" : ObjectId("59ef52ad06b0505f997f84ca"),                        "viewedby" : "user3"                }        ]}