db={"properties":[{"_id":"1","status":"Approved","name":"My Property Lake"}],"property_prices":[{"property_id":"1","block_id":"ABC",status:"Completed","monthly_unit_price":[{"month":"Jan","unit_price":100},{"month":"Dec","unit_price":"1200"}],},{"property_id":"1","block_id":"DEF",status:"Completed","monthly_unit_price":[{"month":"Jan","unit_price":"200"},{"month":"Dec","unit_price":"2400"}],}]}

db.properties.aggregate([{$lookup:{from:"property_prices",as:"property_prices",let:{property_id:"$_id"},pipeline:[{$match:{$expr:{$and:[{$eq:["$property_id","$$property_id"]},{$eq:["$status","Completed"]}]}}}]}},{$addFields:{minInvestment:{$min:{$reduce:{input:{$map:{input:"$property_prices",as:"property",in:{$map:{input:"$$property.monthly_unit_price",as:"price",in:{$toInt:"$$price.unit_price"}}}}},initialValue:[],in:{"$concatArrays":["$$value","$$this"]}}}}}}])