Database
db={"sports":[{_id:1,Name:"tennis"},{_id:2,Name:"soccer"}],"fixture":[{Sport:1,StartDate:ISODate("2022-01-01")},{Sport:1,StartDate:ISODate("2022-01-02")},{Sport:1,StartDate:ISODate("2022-01-03")},{Sport:2,StartDate:ISODate("2022-03-01")}]}
Query
db.sports.aggregate([{$lookup:{from:"fixture",let:{sportId:"$_id"},pipeline:[{$match:{$expr:{$eq:["$Sport","$$sportId"]}}},{$project:{_id:1}}],as:"matchedFixtures"}},{$project:{_id:0,sport:"$Name",sportId:"$_id",fixturesCount:{$size:"$matchedFixtures"}}}])