When I read your post, I made the assumption a user will have 0 to 100s of estates and they can be added in any order so having an arbitrary index to refer to an estate is fragile as it will break if you happened to have “Restaurang” at index 0 for some users.
If you only have a fixed number of estates and the order is fixed (i.e you’re basically treating the array as a object where the key is the index) then rob’s solution will work. vjau’s is a nice syntactic sugar on the client side.
You do need to learn about minimongo and how the data gets transferred to the server and all the security/validation that goes on behind the scenes at some later time though because mongo-extend allows your players to change everything in the estates array.