In Schemas, create a schema using advanced option "import" and attached xsd file that references the given field:
<xs:element name="groupCode" type="xs:string" nillable="false" nxsv:nillable="false"> <xs:simpleType> <xs:restriction base="xs:integer"> <xs:minExclusive value="0" /> </xs:restriction> </xs:simpleType> </xs:element>
The int restriction makes Nuxeo handle this field as a LongProperty, but Studio still parses this field as "string".