Constraint data to be unique according to value of 'field'.
This method allows de-duplication of results from multiple scopes by only allowing unique values for the given field. As an example, a scope that is aggregating results from different local music databases could simply constraint the results on values of the "uri" field, which would ensure that songs with the same uri are displayed only once.
| category_index |
Index of the constrained category, or -1 to constraint all categories |
| field |
Name of the constrained field |