diff --git a/tests/schema/test_core.py b/tests/schema/test_core.py
index d230967..509589c 100644
--- a/tests/schema/test_core.py
+++ b/tests/schema/test_core.py
@@ -104,6 +104,14 @@ class OneToOneLeaf(core.Model):
     root = core.OneToOneAttribute(OneToOneRoot, related_name='leaf')
 
 
+class ManyToOneRoot(core.Model):
+    id = core.SlugAttribute(verbose_name='ID')
+
+
+class ManyToOneLeaf(core.Model):
+    root = core.ManyToOneAttribute(ManyToOneRoot, related_name='leaves')
+
+
 class ManyToManyRoot(core.Model):
     id = core.SlugAttribute(verbose_name='ID')
 
@@ -679,6 +687,128 @@ def test_validate_manytomany_attribute(self):
             error = obj.validate()
             self.assertEqual(error, None)
 
+    def test_onetoone_set_related(self):
+        root = OneToOneRoot()
+        leaf = OneToOneLeaf()
+
+        root.leaf = leaf
+        self.assertEqual(leaf.root, root)
+
+        root.leaf = None
+        self.assertEqual(leaf.root, None)
+
+        leaf.root = root
+        self.assertEqual(root.leaf, leaf)
+
+        leaf.root = None
+        self.assertEqual(root.leaf, None)
+
+    def test_manytoone_set_related(self):
+        roots = [
+            ManyToOneRoot(),
+            ManyToOneRoot(),
+        ]
+        leaves = [
+            ManyToOneLeaf(),
+            ManyToOneLeaf(),
+        ]
+
+        leaves[0].root = roots[0]
+        self.assertEqual(roots[0].leaves, set(leaves[0:1]))
+
+        leaves[1].root = roots[0]
+        self.assertEqual(roots[0].leaves, set(leaves[0:2]))
+
+        leaves[0].root = None
+        self.assertEqual(roots[0].leaves, set(leaves[1:2]))
+
+        roots[0].leaves = set()
+        self.assertEqual(roots[0].leaves, set())
+        self.assertEqual(leaves[1].root, None)
+
+        roots[0].leaves.add(leaves[0])
+        self.assertEqual(roots[0].leaves, set(leaves[0:1]))
+        self.assertEqual(leaves[0].root, roots[0])
+
+        roots[0].leaves.update(leaves[1:2])
+        self.assertEqual(roots[0].leaves, set(leaves[0:2]))
+        self.assertEqual(leaves[1].root, roots[0])
+
+        roots[0].leaves.remove(leaves[0])
+        self.assertEqual(roots[0].leaves, set(leaves[1:2]))
+        self.assertEqual(leaves[0].root, None)
+
+        roots[0].leaves = set()
+        leaves[0].root = roots[0]
+        leaves[0].root = roots[1]
+        self.assertEqual(roots[0].leaves, set())
+        self.assertEqual(roots[1].leaves, set(leaves[0:1]))
+
+        roots[0].leaves = leaves[0:1]
+        self.assertEqual(roots[0].leaves, set(leaves[0:1]))
+        self.assertEqual(roots[1].leaves, set())
+        self.assertEqual(leaves[0].root, roots[0])
+
+        roots[1].leaves = leaves[0:2]
+        self.assertEqual(roots[0].leaves, set())
+        self.assertEqual(roots[1].leaves, set(leaves[0:2]))
+        self.assertEqual(leaves[0].root, roots[1])
+        self.assertEqual(leaves[1].root, roots[1])
+
+    def test_manytomany_set_related(self):
+        roots = [
+            ManyToManyRoot(),
+            ManyToManyRoot(),
+        ]
+        leaves = [
+            ManyToManyLeaf(),
+            ManyToManyLeaf(),
+        ]
+
+        roots[0].leaves.add(leaves[0])
+        self.assertEqual(leaves[0].roots, set(roots[0:1]))
+
+        roots[0].leaves.remove(leaves[0])
+        self.assertEqual(leaves[0].roots, set())
+
+        roots[0].leaves.add(leaves[0])
+        roots[1].leaves.add(leaves[0])
+        self.assertEqual(leaves[0].roots, set(roots[0:2]))
+
+        roots[0].leaves.clear()
+        roots[1].leaves.clear()
+        self.assertEqual(leaves[0].roots, set())
+        self.assertEqual(leaves[1].roots, set())
+
+        roots[0].leaves = leaves
+        roots[1].leaves = leaves
+        self.assertEqual(leaves[0].roots, set(roots[0:2]))
+        self.assertEqual(leaves[1].roots, set(roots[0:2]))
+
+        # reverse
+        roots[0].leaves.clear()
+        roots[1].leaves.clear()
+
+        leaves[0].roots.add(roots[0])
+        self.assertEqual(roots[0].leaves, set(leaves[0:1]))
+
+        leaves[0].roots.remove(roots[0])
+        self.assertEqual(roots[0].leaves, set())
+
+        leaves[0].roots.add(roots[0])
+        leaves[1].roots.add(roots[0])
+        self.assertEqual(roots[0].leaves, set(leaves[0:2]))
+
+        leaves[0].roots.clear()
+        leaves[1].roots.clear()
+        self.assertEqual(roots[0].leaves, set())
+        self.assertEqual(roots[1].leaves, set())
+
+        leaves[0].roots = roots
+        leaves[1].roots = roots
+        self.assertEqual(roots[0].leaves, set(leaves[0:2]))
+        self.assertEqual(roots[1].leaves, set(leaves[0:2]))
+
     def test_clean_and_validate_objects(self):
         grandparent = Grandparent(id='root')
         parents = [
diff --git a/tests/schema/test_io.py b/tests/schema/test_io.py
index 6103cc6..fcb55da 100644
--- a/tests/schema/test_io.py
+++ b/tests/schema/test_io.py
@@ -23,7 +23,7 @@ class Meta(core.Model.Meta):
 
 
 class Node(core.Model):
-    id = core.StringAttribute(primary=True, unique=True)
+    id = core.SlugAttribute(primary=True)
     root = core.ManyToOneAttribute(Root, related_name='nodes')
     val1 = core.FloatAttribute()
     val2 = core.FloatAttribute()
@@ -34,12 +34,12 @@ class Meta(core.Model.Meta):
 
 class Leaf(core.Model):
     id = core.StringAttribute(primary=True)
-    node = core.ManyToOneAttribute(Node, related_name='leaves')
+    nodes = core.ManyToManyAttribute(Node, related_name='leaves')
     val1 = core.FloatAttribute()
     val2 = core.FloatAttribute()
 
     class Meta(core.Model.Meta):
-        attribute_order = ('id', 'node', 'val1', 'val2', )
+        attribute_order = ('id', 'nodes', 'val1', 'val2', )
 
 
 class TestIo(unittest.TestCase):
@@ -54,17 +54,17 @@ def tearDown(self):
     def test_write_read(self):
         root = Root(id='root', name='root')
         nodes = [
-            Node(root=root, id='node-0', val1=1, val2=2),
+            Node(root=root, id=u'node-0-\u20ac', val1=1, val2=2),
             Node(root=root, id='node-1', val1=3, val2=4),
             Node(root=root, id='node-2', val1=5, val2=6),
         ]
         leaves = [
-            Leaf(node=nodes[0], id='leaf-0-0', val1=7, val2=8),
-            Leaf(node=nodes[0], id='leaf-0-1', val1=9, val2=10),
-            Leaf(node=nodes[1], id='leaf-1-0', val1=11, val2=12),
-            Leaf(node=nodes[1], id='leaf-1-1', val1=13, val2=14),
-            Leaf(node=nodes[2], id='leaf-2-0', val1=15, val2=16),
-            Leaf(node=nodes[2], id='leaf-2-1', val1=17, val2=18),
+            Leaf(nodes=[nodes[0]], id='leaf-0-0', val1=7, val2=8),
+            Leaf(nodes=[nodes[0]], id='leaf-0-1', val1=9, val2=10),
+            Leaf(nodes=[nodes[1]], id='leaf-1-0', val1=11, val2=12),
+            Leaf(nodes=[nodes[1]], id='leaf-1-1', val1=13, val2=14),
+            Leaf(nodes=[nodes[2]], id='leaf-2-0', val1=15, val2=16),
+            Leaf(nodes=[nodes[2]], id='leaf-2-1', val1=17, val2=18),
         ]
 
         objects = set((root, )) | root.get_related()
@@ -79,3 +79,6 @@ def test_write_read(self):
 
         root2 = objects2[Root].pop()
         self.assertEqual(root2, root)
+
+        # unicode
+        self.assertEqual(next(obj for obj in objects2[Node] if obj.val1 == 1).id, u'node-0-\u20ac')
diff --git a/tests/schema/test_utils.py b/tests/schema/test_utils.py
index bed7e8a..e1d0d38 100644
--- a/tests/schema/test_utils.py
+++ b/tests/schema/test_utils.py
@@ -5,6 +5,7 @@
 :Copyright: 2016, Karr Lab
 :License: MIT
 """
+from six import string_types
 from wc_utils.schema import core, utils
 import sys
 import unittest
@@ -72,4 +73,4 @@ def test_get_related_errors(self):
         self.assertEqual(len(errors_by_model[Root]), 1)
         self.assertEqual(len(errors_by_model[Node]), 2)
 
-        self.assertIsInstance(utils.get_object_set_error_string(errors), str)
+        self.assertIsInstance(utils.get_object_set_error_string(errors), string_types)
diff --git a/wc_utils/schema/core.py b/wc_utils/schema/core.py
index bdd5410..c1a76d6 100644
--- a/wc_utils/schema/core.py
+++ b/wc_utils/schema/core.py
@@ -121,7 +121,7 @@ def init_related_attributes(cls):
                                 related_class.Meta.primary_attribute.name, related_class.__name__))
 
                         if isinstance(attr, ManyToManyAttribute) and not isinstance(related_class.Meta.primary_attribute, (SlugAttribute, IntegerAttribute)):
-                            raise ValueError('Primary attribute {} of related class {} must be unique'.format(
+                            raise ValueError('Primary attribute {} of related class {} must be a slug or integer attribute'.format(
                                 related_class.Meta.primary_attribute.name, related_class.__name__))
 
                         # check that name doesn't conflict with another attribute
@@ -1762,7 +1762,7 @@ def deserialize(self, value, objects):
             :obj:`tuple` of `object`, `InvalidAttribute`: tuple of cleaned value and cleaning error
         """
         if not value:
-            return None
+            return (None, None)
 
         related_objs = []
         related_classes = chain([self.related_class], get_subclasses(self.related_class))
@@ -1932,7 +1932,7 @@ def deserialize(self, value, objects):
             :obj:`tuple` of `object`, `InvalidAttribute`: tuple of cleaned value and cleaning error
         """
         if not value:
-            return None
+            return (None, None)
 
         related_objs = []
         related_classes = chain([self.related_class], get_subclasses(self.related_class))
@@ -2098,7 +2098,7 @@ def deserialize(self, values, objects):
             :obj:`tuple` of `object`, `InvalidAttribute`: tuple of cleaned value and cleaning error
         """
         if not values:
-            return set()
+            return (set(), None)
 
         deserialized_values = set()
         errors = []
@@ -2156,7 +2156,7 @@ def discard(self, value):
 
     def clear(self):
         """ Remove all elements from set """
-        for value in self:
+        for value in list(self):
             self.remove(value)
 
     def pop(self):
@@ -2179,7 +2179,7 @@ def intersection_update(self, values):
         Args:
             values (:obj:`set`): values to intersect with set
         """
-        for value in self:
+        for value in list(self):
             if value not in values:
                 self.remove(value)
 
@@ -2199,12 +2199,12 @@ def symmetric_difference_update(self, values):
         Args:
             values (:obj:`set`): values to difference with set
         """
-        for value in self:
+        for value in list(self):
             if value in values:
                 self.remove(value)
                 values.remove(value)
 
-        for value in values:
+        for value in list(values):
             if value in self:
                 self.remove(value)
                 values.remove(value)
@@ -2224,7 +2224,7 @@ def add(self, value, propagate=True):
         """
         super(ManyToOneRelatedManager, self).add(value)
         if propagate:
-            setattr(value, attr.name, obj)
+            value.__setattr__(self.attribute.name, self.object)
 
     def remove(self, value, update_set=True, propagate=True):
         """ Remove value from set
@@ -2233,10 +2233,10 @@ def remove(self, value, update_set=True, propagate=True):
             value (:obj:`object`): value
             propagate (:obj:`bool`, optional): propagate change to related attribute
         """
-        if update_set:
+        if update_set and value in self:
             super(ManyToOneRelatedManager, self).remove(value)
         if propagate:
-            setattr(value, attr.name, None)
+            value.__setattr__(self.attribute.name, None)
 
 
 class ManyToManyRelatedManager(RelatedManager):
@@ -2269,9 +2269,9 @@ def add(self, value, propagate=True):
         super(ManyToManyRelatedManager, self).add(value)
         if propagate:
             if self.related:
-                getattr(value, self.attribute.name).add(self.object)
+                getattr(value, self.attribute.name).add(self.object, propagate=False)
             else:
-                getattr(value, self.attribute.related_name).add(self.object)
+                getattr(value, self.attribute.related_name).add(self.object, propagate=False)
 
     def remove(self, value, update_set=True, propagate=True):
         """ Remove value from set
@@ -2285,9 +2285,9 @@ def remove(self, value, update_set=True, propagate=True):
             super(ManyToManyRelatedManager, self).remove(value)
         if propagate:
             if self.related:
-                getattr(value, self.attribute.name).remove(self.object)
+                getattr(value, self.attribute.name).remove(self.object, propagate=False)
             else:
-                getattr(value, self.attribute.related_name).remove(self.object)
+                getattr(value, self.attribute.related_name).remove(self.object, propagate=False)
 
 
 class InvalidObjectSet(object):
diff --git a/wc_utils/schema/utils.py b/wc_utils/schema/utils.py
index 31deadf..5fe2e04 100644
--- a/wc_utils/schema/utils.py
+++ b/wc_utils/schema/utils.py
@@ -7,6 +7,7 @@
 """
 
 # todo: add method to compare (difference) models
+from __future__ import unicode_literals
 from wc_utils.schema.core import Model, Attribute, RelatedAttribute, InvalidObjectSet, InvalidObject, clean_and_validate_objects