db={"rows_items":[{"tableName":"Items","label":"row 1 label here","item":"item1"},{"tableName":"Items","label":"row 1 label here","item":"item2"},{"tableName":"Items","label":"row 1 label here","item":"item3"},{"tableName":"Items","label":"row 2 label here","item":"item4"},{"tableName":"Items","label":"row 2 label here","item":"item5"},{"tableName":"Items","label":"row 2 label here","item":"item6"}],"items":[{"id":"item1","name":"Item name","code":"XXX-XXXX-XXXX","price":100},{"id":"item2","name":"Item name","code":"XXX-XXXX-XXXX","price":100},{"id":"item3","name":"Item name","code":"XXX-XXXX-XXXX","price":100}]}

db.rows_items.aggregate([{"$lookup":{"from":"items","localField":"item","foreignField":"id","as":"itemsLookup"}},{"$unwind":"$itemsLookup"},{"$replaceRoot":{"newRoot":{"$mergeObjects":["$$ROOT","$itemsLookup"]}}},{"$unset":"itemsLookup"}])