db={posts:[{_id:1,createdAt:ISODate("2021-01-01"),},{_id:2,createdAt:ISODate("2021-01-02"),}],photos:[{_id:"1234",url:"some.url.com",published:true,postId:1},{_id:"4567",url:"some.otherurl.com",published:false,postId:2},]}

db.posts.aggregate([{"$lookup":{"from":"photos",let:{postId:"$_id"},pipeline:[{$match:{$expr:{$and:[{$eq:["$postId","$$postId"]},{$eq:[false,"$published"]}]}}},],"as":"photoData"}},{"$project":{"createdAt":1,"photoData":1}}])