Here's the how I append data to struct:
user.Things = append(user.Things, item.Id)
Now, how can I remove item.id from user.Things? Seems like there's no method like delete, remove, or similar.
For example, this didn't work:
user.Things = append(user.Things[:item.id], user.Things[:item.id + 1:])
The wiki page Slice tricks gives a good overview about operations on slices.
There is also a few ways to delete an elements a slice: cutting, deleting or deleting without preserving order.
In your case, it seems you just had a typo (an extra colon):
user.Things = append(user.Things[:item.id], user.Things[item.id + 1:])