Database
[{"_id":{"$oid":"5fbfa0005c15aaf2eac69ba6"},"PostMedia":[{"FilePath":"http://localhost:8886/localhost44323/image/","Filename":"img1.jpg","Title":null,"Description":""},{"FilePath":"http://localhost:8886/localhost44323/image/","Filename":"img2.jpg","Title":null,"Description":""}]},{"_id":{"$oid":"5fbfa0485c15aaf2eac69ba7"},"PostMedia":[{"FilePath":"http://localhost:8886/localhost44323/image/","Filename":"img3.jpg","Title":null,"Description":""},{"FilePath":"http://localhost:8886/localhost44323/image/","Filename":"img4.jpg","Title":null,"Description":""}]}]
Query
db.collection.aggregate([{$unwind:"$PostMedia"},{$group:{_id:null,PostMedia:{$push:"$PostMedia"}}},{$unset:"_id"}])