How to apply condition on lookup result in mongoDB

questionsHow to apply condition on lookup result in mongoDB
Chandresh asked 2 years ago

Hi 
i have two collections
Photo:

{_id: 1, photo_name: '1.jpg', photo_description: 'description 1', album_id: 1, flag:1 },
{_id: 2, photo_name: '2.jpg', photo_description: 'description 2', album_id: 1, flag:1 },
{_id: 3, photo_name: '3.jpg', photo_description: 'description 3', album_id: 1, flag:1 },
{_id: 4, photo_name: '4.jpg', photo_description: 'description 4', album_id: 2, flag:0 },
{_id: 5, photo_name: '5.jpg', photo_description: 'description 5', album_id: 2, flag:0 },
{_id: 6, photo_name: '6.jpg', photo_description: 'description 6', album_id: 2}


Album:

{_id: 1, album_name: "my album 1", album_description: "album description 1", emoji_id: 1},
{_id: 2, album_name: "my album 2", album_description: "album description 2", emoji_id: 2},
{_id: 3, album_name: "my album 3", album_description: "album description 3", emoji_id: 3},
{_id: 4, album_name: "my album 4", album_description: "album description 4", emoji_id: 4},
{_id: 5, album_name: "my album 5", album_description: "album description 5", emoji_id: 5}
 

Suppose I add one another field of flag in photo collection and now I want to get count only those photos whose flag is one.
I tried to add $match immediately after $lookup in query, but it fails, It doesn’t exclude photos whose flag=0 and also in counter it does not flag condition.