[rset] New method: ResultSet.one()
This method will return exactly one entity while enforcing its existence and unicity.
The idea is shamelessly borowed from SQLAlchemy Query.one().
Closes #3352314
[jcr: use len(self) instead of len(self.rows)]
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# This file is part of CubicWeb.
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"""mapping file for source used in unittest_multisources.py"""
support_entities = {'Card': True, 'Affaire': True, 'State': True}
support_relations = {'in_state': True, 'documented_by': True, 'multisource_inlined_rel': True}
cross_relations = set( ('documented_by',) )