--- a/_exceptions.py	Tue Dec 10 12:36:50 2013 +0100
+++ b/_exceptions.py	Wed Dec 11 17:52:54 2013 +0100
@@ -134,6 +134,15 @@
     a non final entity
     """
 
+class MultipleResultsError(CubicWebRuntimeError):
+    """raised when ResultSet.one() is called on a resultset with multiple rows
+    of multiple columns.
+    """
+
+class NoResultError(CubicWebRuntimeError):
+    """raised when no result is found but at least one is expected.
+    """
+
 class UndoTransactionException(QueryError):
     """Raised when undoing a transaction could not be performed completely.
 

--- a/rset.py	Tue Dec 10 12:36:50 2013 +0100
+++ b/rset.py	Wed Dec 11 17:52:54 2013 +0100
@@ -23,7 +23,7 @@
 
 from rql import nodes, stmts
 
-from cubicweb import NotAnEntity
+from cubicweb import NotAnEntity, NoResultError, MultipleResultsError
 
 
 class ResultSet(object):
@@ -437,6 +437,25 @@
             raise NotAnEntity(etype)
         return self._build_entity(row, col)
 
+    def one(self, col=0):
+        """Retrieve exactly one entity from the query.
+
+        If the result set is empty, raises :exc:`NoResultError`.
+        If the result set has more than one row, raises
+        :exc:`MultipleResultsError`.
+
+        :type col: int
+        :param col: The column localising the entity in the unique row
+
+        :return: the partially initialized `Entity` instance
+        """
+        if len(self) == 1:
+            return self.get_entity(0, col)
+        elif len(self) == 0:
+            raise NoResultError("No row was found for one()")
+        else:
+            raise MultipleResultsError("Multiple rows were found for one()")
+
     def _build_entity(self, row, col):
         """internal method to get a single entity, returns a partially
         initialized Entity instance.

--- a/test/unittest_rset.py	Tue Dec 10 12:36:50 2013 +0100
+++ b/test/unittest_rset.py	Wed Dec 11 17:52:54 2013 +0100
@@ -28,6 +28,8 @@
 from cubicweb.devtools.testlib import CubicWebTC
 from cubicweb.rset import NotAnEntity, ResultSet, attr_desc_iterator
 
+from cubicweb import NoResultError, MultipleResultsError
+
 
 def pprelcachedict(d):
     res = {}
@@ -368,6 +370,39 @@
             attr = etype == 'Bookmark' and 'title' or 'name'
             self.assertEqual(entity.cw_attr_cache[attr], n)
 
+    def test_one(self):
+        self.request().create_entity('CWUser', login=u'cdevienne',
+                                     upassword=u'cdevienne',
+                                     surname=u'de Vienne',
+                                     firstname=u'Christophe')
+        e = self.execute('Any X WHERE X login "cdevienne"').one()
+
+        self.assertEqual(e.surname, u'de Vienne')
+
+        e = self.execute(
+            'Any X, N WHERE X login "cdevienne", X surname N').one()
+        self.assertEqual(e.surname, u'de Vienne')
+
+        e = self.execute(
+            'Any N, X WHERE X login "cdevienne", X surname N').one(col=1)
+        self.assertEqual(e.surname, u'de Vienne')
+
+    def test_one_no_rows(self):
+        with self.assertRaises(NoResultError):
+            self.execute('Any X WHERE X login "patanok"').one()
+
+    def test_one_multiple_rows(self):
+        self.request().create_entity(
+            'CWUser', login=u'cdevienne', upassword=u'cdevienne',
+            surname=u'de Vienne', firstname=u'Christophe')
+
+        self.request().create_entity(
+            'CWUser', login=u'adim', upassword='adim', surname=u'di mascio',
+            firstname=u'adrien')
+
+        with self.assertRaises(MultipleResultsError):
+            self.execute('Any X WHERE X is CWUser').one()
+
     def test_related_entity_optional(self):
         e = self.request().create_entity('Bookmark', title=u'aaaa', path=u'path')
         rset = self.execute('Any B,U,L WHERE B bookmarked_by U?, U login L')