bigfish24
commented
6 years ago If your objects can have primary keys then you can utilize Realm's merge logic to achieve the goal.
Another neat feature of Realm is that you can create objects on two devices with the same primary key and they will ultimately merge once synced into just having one object.
The only thing to keep in mind is how the merge of these two objects happen. Take for example a Settings object that stores the user's preferences which you want to sync across devices. If one devices creates it and then adjust a setting, while at the same time another device creates the object but doesn't yet change anything, you would want the sync to occur such that the object that has user driven changes "wins". The merge algorithm supports this if you make sure to create the objects with default values and then apply changes to it if the user does so. The default values will be overridden by other changes even if those changes occurred earlier.
For Swift this can be a little tricky because you declare default values in the schema in such a way we can't know that they are default:
@objc dynamic var name = "" // default empty stringAs a result, with Swift you need to specifically use the realm.create API:
You can also partially update objects with primary keys by passing just a subset of the values you wish to update, along with the primary key:
// Assuming a "Book" with a primary key of `1` already exists. try! realm.write { realm.create(Book.self, value: ["id": 1, "price": 9000.0], update: true) // the book's `title` property will remain unchanged. }Any property you leave out from the dictionary you pass in will then use the default value and we know it is default, thus the merge semantics are as described above.
Hope this helps!
ivnsch
trant
astigsen
Goals
Add items with a semantic unique (edit: i.e. not the primary key) in 2 different devices at the same time.
Expected Results
Only 1 of them is added - the other is dismissed ( /deleted) as a duplicate.
Actual Results
I get duplicates.
Description
This is rather a general question - what can I do to avoid or fix this?
The uniques are composed by fields, some of which are in nested objects.
Since Realm doesn't seem to support this kind of uniques, the unique isn't recognized and the remote database just adds 2x the item.
I tried implementing a consistency check in the notification block, so when it detects a duplicate it removes it. The problem with this was that it ended removing both items, as each device deleted the item of the other device!
In my particular case, for diverse reasons, this is not a rare case, so I have to handle it somehow. In more detail: Users can swipe items from a
todolist to adonelist and it can happen that they swipe an item at the same time. My current logic (which I can't change at this point) checks (locally) whether there's an item already with the same semantic unique in thedonelist, if yes increments it, otherwise creates a new item. So when multiple users swipe the element at the same time and it's not yet in thedoneeach will create a new one locally and upload this to the ROS.