cursor.One should work without checking IsNil on a user side.

`cursor.One` is a higher level funcion to get a single element from the result
 set. It already returns `ErrEmptyResult`, so to be consistent with a
function responsibility it should also return that error even when user
didn't check `IsNil` before performing `One`.

Author: Robert Zaremba

query_control_test.go

@@ -15,7 +15,7 @@ func (s *RethinkSuite) TestControlExecNil(c *test.C) {
 
 	err = res.One(&response)
 
-	c.Assert(err, test.IsNil)
+	c.Assert(err, test.Equals, ErrEmptyResult)
 	c.Assert(response, test.Equals, nil)
 }
 

results.go

@@ -199,6 +199,9 @@ func (c *Cursor) All(result interface{}) error {
 // One retrieves a single document from the result set into the provided
 // slice and closes the cursor.
 func (c *Cursor) One(result interface{}) error {
+	if c.IsNil() {
+		return ErrEmptyResult
+	}
 	ok := c.Next(result)
 	if !ok {
 		err := c.Err()

results_test.go

@@ -163,7 +163,8 @@ func (s *RethinkSuite) TestEmptyResults(c *test.C) {
 	res, err = Db("test").Table("test").Get("missing value").Run(sess)
 	c.Assert(err, test.IsNil)
 	var response interface{}
-	res.One(&response)
+	err = res.One(&response)
+	c.Assert(err, test.Equals, ErrEmptyResult)
 	c.Assert(res.IsNil(), test.Equals, true)
 
 	res, err = Db("test").Table("test").Get("missing value").Run(sess)