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Now that we have a class, which is stored in the DB we might like
to add a field and constrain it to certain values. An example would
be if we added a field called age which was an int, this field must be
= 0 so an entry to place this constraint looks like this:
<field name="age" type="integer">
<sql name="user_age" constraint-name="user_age_gte_zero"
This code will add a constraint on age such that an exception is
thrown and the operation not performed if someone tries to set an age
less than zero.