Add constraint that ensures that a key field is unique in a collection

[hisystems]

A key field is typically a value that a user can enter and modify. For example, a product code from a products table. This constraint should ensure:

• if the object is new, that the key value has not already been used in the table / table subset
• if the value has changed, then the new value does not conflict with an existing value already in the table / table subset.
  This constraint should encapsulate this concept and eliminate the need to repeatedly resolve this problem for different tables and scenarios.
• if the value has not changed then the constraint passes