How to get documents by tags in MongoDB?

You can use $elemMatch operator for this. Let us create a collection with documents

> db.getDocumentsByTagsDemo.insertOne({"Tags":["Tag-1", "Tag-2", "Tag-3"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9eb4d5d628fa4220163b79") } > db.getDocumentsByTagsDemo.insertOne({"Tags":["Tag-2", "Tag-4", "Tag-5"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9eb4d5d628fa4220163b7a") } > db.getDocumentsByTagsDemo.insertOne({"Tags":["Tag-6", "Tag-4", "Tag-3"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5c9eb4d6d628fa4220163b7b") }

Following is the query to display all documents from a collection with the help of find() method

> db.getDocumentsByTagsDemo.find().pretty();

This will produce the following output

{    "_id" : ObjectId("5c9eb4d5d628fa4220163b79"),    "Tags" : [       "Tag-1",       "Tag-2",       "Tag-3"    ] } {    "_id" : ObjectId("5c9eb4d5d628fa4220163b7a"),    "Tags" : [       "Tag-2",       "Tag-4",       "Tag-5"    ] } {    "_id" : ObjectId("5c9eb4d6d628fa4220163b7b"),    "Tags" : [       "Tag-6",       "Tag-4",       "Tag-3"    ] }

Following is the query to get documents by tags

> db.getDocumentsByTagsDemo.find({Tags: { $elemMatch: { $eq: "Tag-2" } }}).pretty();

This will produce the following output

{    "_id" : ObjectId("5c9eb4d5d628fa4220163b79"),    "Tags" : [       "Tag-1",       "Tag-2",       "Tag-3"    ] } {    "_id" : ObjectId("5c9eb4d5d628fa4220163b7a"),    "Tags" : [       "Tag-2",       "Tag-4",       "Tag-5"    ] }

Following is the query to find specific tags only

> db.getDocumentsByTagsDemo.find({Tags: "Tag-5"}).pretty();

This will produce the following output

{    "_id" : ObjectId("5c9eb4d5d628fa4220163b7a"),    "Tags" : [       "Tag-2",       "Tag-4",       "Tag-5"    ] }