Database
db={countryTable:[{"_id":ObjectId("627cd43f48aea72fdc0d88e0"),"county":"india"},{"_id":ObjectId("627cd43f48aea72fdc0d88e1"),"county":"china"}],cityTable:[{"_id":ObjectId("627cd43f48aea72fdc0d88e0"),"county_name":"India"},{"_id":ObjectId("627cd43f48aea72fdc0d88e1"),"county_name":"China"}]}
Query
db.countryTable.aggregate([{$lookup:{from:"cityTable",let:{county:"$county"},pipeline:[{$match:{"$expr":{"$regexMatch":{"input":"$county_name","regex":"$$county","options":"i"}}}}],as:"citydetails"}},{$unwind:"$citydetails"}])