Sets the name of the referential integrity constraint for a field or disables the constraint. If the value attribute of the extension is "{no}" then no constraint is generated. Some other value is used as the name of the constraint instead of the default name provided by the name generator for the datastore. Currently this extension may be used for fields that reference persistent classes. This extension is ignored if the field allows nulls and the target database does not support referential integrity constraints on columns that allow nulls.
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<field name="category">
<extension key="db-constraint" value="ref_cat" />
</field>
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