Database
db={"france":[{field1:11,field2:12,field3:13},{field1:21,field2:22,field3:23},{field1:31,field2:32,field3:33}],"germany":[{field1:11,field2:12,field3:13},{field1:21,field2:22,field3:23},{field1:31,field2:32,field3:33}]}
Query
db.france.aggregate([{$group:{_id:null,france:{$push:"$$ROOT"}}},{$lookup:{from:"germany",pipeline:[],as:"germany"}},{$project:{root:{$concatArrays:["$france","$germany"]}}},{$unwind:"$root"},{$replaceWith:"$root"}])