Database
[{_id:7698,name:"Blake",job:"manager",manager:7839,hired:"1981-05-01T00:00:00.000Z",salary:2850,department:{name:"Sales",location:"Chicago"},missions:[{company:"Mac Donald",location:"Chicago"},{company:"IBM",location:"Chicago"}]}]
Query
db.collection.aggregate([{$match:{"missions":{$exists:true}}},{$project:{name:1,nbMissionsChicago:{$eq:[{$filter:{input:"$missions",cond:{$eq:["$$this.location","Chicago"]}}},"$missions"]}}}])