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androidsqliteandroid-contentprovider

Batch job - violate constraint temporarily

Let's think about a column with a unique constraint and following values:

Col1
1
2
3

Now I want to change those values in a batch job to following:

Col1
2
3
4

The result is, as soon as I change 1 to 2, this violates the unique constraint and the batch job is stopped. Although, after finishing the batch job, the constraint would be fulfilled again.

How do I solve that problem?

Use case

I'm trying to rename a list of files in the media store via contentResolver.applyBatch(MediaStore.AUTHORITY, operations) and this throws an constraint violation exception...

Solution

  • In such case that renaming elements might cause collisions on a unique column, you need to apply the transformation in a safe order.

    Basically instead of asking the content provider to perform the following operations 

    rename 1 → 2
rename 2 → 3
rename 3 → 4

    You should ask 

    rename 3 → 4
rename 2 → 3
rename 1 → 2

    Which always satisfies the unique constraint.

    Also even without the unique constraint, the first method would potentially rename everything to 4