Core Data Entity Unique Constraint Does Not Work

Core Data Entity Unique Constraint Does Not Work

If you want to get an error when there are merge conflicts and handle them manually then you need to change your policy to NSErrorMergePolicy and you will get an error and in the user info the object IDs that you need to solve merge conflict , otherwise it will merge and save according to the specified merge policy.
The policy that you set will overwrite the object attributes but not relationships, if you want to overwrite the attributes and relationships then specify the NSMergeByPropertyObjectTrumpMergePolicy.

Unique Constraints do not work

Unique Constraints only work when you save the context.

After creating NSMangaedObjects save that NSManagedContext in which they were created.

Yes unique constraints doesnot work if you have relationship in your entity.(I tried this with xcode 7.3 ios9)

Core Data Unique constraint except for the default value

Core Data's uniqueness constraints are just that-- they require uniqueness, without exceptions. Your situation of wanting "unique except for one value that can be duplicated" isn't directly supported by Core Data. You'd have to maintain that in your own code somehow. That probably means implementing your own update-or-insert logic to check whether an ID exists. That would be something like,

1. Do a fetch with the specific ID.
2. If you find an object update it.
3. If you don't find an object, create a new one.

CoreData unique constraints disappears in Xcode

I believe this to be a bug in Xcode. If you change the model to add (or amend or remove) a constraint, the underlying model file is not updated, even though the screen shows the changes correctly. To overcome it, change another element of the model at the same time as adding the uniqueness constraint. Save the model. Then change the other element back and save again.

core data ios9: multifield unique constraint

It is possible to set uniqueness constraints for a combination of attributes. You were on the right path putting both attributes on the same line in the constraints:

You may have found this had no effect due to a problem with Xcode: the constraints are not actually updated in the model unless you modify some other aspect of the model at the same time (eg. change an attribute type and then change it back).

If you look at the SQL being generated, the table is created with a constraint:

CREATE TABLE ZSTOREOBJECT ( Z_PK INTEGER PRIMARY KEY, Z_ENT INTEGER, Z_OPT INTEGER, ...., CONSTRAINT ZLASTNAME_ZFIRSTNAME UNIQUE (ZLASTNAME, ZFIRSTNAME))

and when you insert new insert new values which fail this constraint (when the context is saved):

CoreData: sql: COMMIT
CoreData: sql: BEGIN EXCLUSIVE
CoreData: sql: INSERT INTO ZSTOREOBJECT(Z_PK, Z_ENT, Z_OPT) VALUES(?, ?, ?)
CoreData: sql: UPDATE ZSTOREOBJECT SET ZLASTNAME = ?, ZFIRSTNAME = ? WHERE Z_PK = ?
CoreData: sql: ROLLBACK

and the error returned is:

Error Domain=NSCocoaErrorDomain Code=133021 "(null)" UserInfo={conflictList=(
"NSConstraintConflict (0x7fbd18d33c10) for constraint (\n    lastName,\n    firstName\n): ....

(This works in Xcode 7.2.1, with the iOS9.2 simulator; I haven't checked prior versions).

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