[1.8.x] Fixed #24505 -- Fixed clash with hidden m2m fields.

Added support for multiple m2m fields with the same 'to' model
and with related_name set to '+'.

Backport of 4ee08958f1 from master

tests/m2m_regress/models.py

@@ -54,9 +54,13 @@ class SelfReferChildSibling(SelfRefer):
 
 
 # Many-to-Many relation between models, where one of the PK's isn't an Autofield
+@python_2_unicode_compatible
 class Line(models.Model):
     name = models.CharField(max_length=100)
 
+    def __str__(self):
+        return self.name
+
 
 class Worksheet(models.Model):
     id = models.CharField(primary_key=True, max_length=100)
@@ -87,3 +91,10 @@ class RegressionModelSplit(BadModelWithSplit):
     Model with a split method should not cause an error in add_lazy_relation
     """
     others = models.ManyToManyField('self')
+
+
+# Regression for #24505 -- Two ManyToManyFields with the same "to" model
+# and related_name set to '+'.
+class Post(models.Model):
+    primary_lines = models.ManyToManyField(Line, related_name='+')
+    secondary_lines = models.ManyToManyField(Line, related_name='+')