Check if combination of values is already entered, using Repeating Controls

Several posts explains how to use drop down list boxes in repeating tables, assuring that the users can only select values which are not selected in previous rows.  This article describes how to extend this validation, to assure the combination of several fields are not entered in previous rows.

Solution:

1. Within the repeating node, add ahidden TextBox, called UniqueValue
2. Set the Default Value to Concat(field1, field2,..)    Field 1, Field 2,... are the fields that will make the row unique
3. Add the following rule to the newly created field:
       Condition: The Expression
       Expression: ../my:UniqueValue = (../preceding-
       sibing::my:Group2 | ../following-
       sibling::my:Group2)/my:UniqueValue)

    Action: Show Dialog box with some message that the
     combination of fields already exists

Group2: is the name of the repeating node within the data-source