Database

db={"Films":[{"_id":{"$oid":"6272aa886441c51b18de7b23"},"type":"Film","title":"Les 2 papas et la maman","genres":"Comedia","description":"Jérôme and Delphine want a child but Jerome is sterile. They then ask the best friend of Jerome, Salim, to be the donor for artificial insemination of the mother...","platform":"Netflix","film_url":"exampleurl1.com"},{"_id":{"$oid":"6272aa886441c51b18de7b24"},"type":"Film","title":"Terminator II","genres":"SciFi","description":"Terminator continued, and better...","platform":"Netflix","film_url":"exampleurl2.com"},],"Actors":[{"_id":{"$oid":"6272ac5b6441c51b18de9ee4"},"name":"Sophie Rundle","film_title":"Peaky Blinders","film_url":"exampleurl.com","character":"Ada Shelby","num_episodes":"36"},{"_id":{"$oid":"6272ac5b6441c51b18de9ee5"},"film_url":"exampleurl1.com"},{"_id":{"$oid":"6272ac5b6441c51b18de9ee6"},"film_url":"exampleurl2.com"},{"_id":{"$oid":"6272ac5b6441c51b18de9ee7"},"film_url":"exampleurl3.com"},{"_id":{"$oid":"6272ac5b6441c51b18de9ee8"},"film_url":"exampleurl1.com"},{"_id":{"$oid":"6272ac5b6441c51b18de9ee9"},"film_url":"exampleurl2.com"},{"_id":{"$oid":"6272ac5b6441c51b18de9eea"},"film_url":"exampleurl3.com"},{"_id":{"$oid":"6272ac5b6441c51b18de9eeb"},"film_url":"exampleurl1.com"}]}

Query

db.Films.aggregate([{"$lookup":{"from":"Actors","localField":"film_url","foreignField":"film_url","pipeline":[{"$project":{"_id":1}}],"as":"actorIds"}},{"$set":{"actorIds":{"$map":{"input":"$actorIds","in":"$$this._id"}}}},{"$merge":"Films"}])

Result