diff --git a/viewflow/fields.py b/viewflow/fields.py
new file mode 100644
index 00000000..27deae26
--- /dev/null
+++ b/viewflow/fields.py
@@ -0,0 +1,138 @@
+import datetime
+import decimal
+import json
+import uuid
+from typing import List
+from django.db import DEFAULT_DB_ALIAS, models, router
+from django.db.models import signals
+from django.db.models.sql.where import WhereNode, AND
+from django.utils.duration import duration_iso_string
+from django.utils.functional import Promise
+from django.utils.timezone import is_aware
+
+
+class CompositeKey(models.AutoField):
+
+    class Key(dict):
+        """Dictionary with json-compatible string conversion."""
+        def __str__(self):
+            return json.dumps(self)
+
+        def __hash__(self):
+            return hash(
+                tuple(self[key] for key in sorted(self.keys()))
+            )
+
+    def __init__(self, columns: List[str], **kwargs):
+        self.columns = columns
+        super().__init__(primary_key=True, **kwargs)
+
+    def contribute_to_class(self, cls, name, private_only=False):
+        self.set_attributes_from_name(name)
+        self.model = cls
+        self.concrete = False
+        self.editable = False
+        self.column = self.columns[0]            # for default order_by
+        cls._meta.add_field(self, private=True)  # virtual field
+        cls._meta.setup_pk(self)                 # acts as pk
+
+        if not getattr(cls, self.attname, None):
+            setattr(cls, self.attname, self)
+
+        def delete(inst, using=None, keep_parents=False):
+            using = using or router.db_for_write(self.model, instance=inst)
+
+            signals.pre_delete.send(
+                sender=cls, instance=inst, using=using
+            )
+
+            query = cls._default_manager.filter(**self.__get__(inst))
+            query._raw_delete(using)
+
+            for column in self.columns:
+                setattr(inst, column, None)
+
+            signals.post_delete.send(
+                sender=cls, instance=inst, using=using
+            )
+
+        cls.delete = delete
+
+    def get_prep_value(self, value):
+        return self.to_python(value)
+
+    def to_python(self, value):
+        if value is None or isinstance(value, CompositeKey.Key):
+            return value
+        return CompositeKey.Key(json.loads(value))
+
+    def to_json(self, value):
+        if isinstance(value, datetime.datetime):
+            result = value.isoformat()
+            if value.microsecond:
+                result = result[:23] + result[26:]
+            if result.endswith('+00:00'):
+                result = result[:-6] + 'Z'
+            return result
+        elif isinstance(value, datetime.date):
+            return value.isoformat()
+        elif isinstance(value, datetime.time):
+            if is_aware(value):
+                raise ValueError("JSON can't represent timezone-aware times.")
+            result = value.isoformat()
+            if value.microsecond:
+                result = result[:12]
+            return result
+        elif isinstance(value, datetime.timedelta):
+            return duration_iso_string(value)
+        elif isinstance(value, (decimal.Decimal, uuid.UUID, Promise)):
+            return str(value)
+        return value
+
+    def bulk_related_objects(self, objs, using=DEFAULT_DB_ALIAS):
+        return []
+
+    def __get__(self, instance, cls=None):
+        if instance is None:
+            return self
+
+        return CompositeKey.Key({
+            column: self.to_json(
+                self.model._meta.get_field(column).value_from_object(instance)
+            )
+            for column in self.columns
+        })
+
+    def __set__(self, instance, value):
+        """
+        I hope it's safe to ignore!
+        """
+        pass
+
+
+@CompositeKey.register_lookup
+class Exact(models.Lookup):
+    lookup_name = 'exact'
+
+    def as_sql(self, compiler, connection):
+        fields = [
+            self.lhs.field.model._meta.get_field(column)
+            for column in self.lhs.field.columns
+        ]
+
+        lookup_classes = [
+            field.get_lookup('exact')
+            for field in fields
+        ]
+
+        lookups = [
+            lookup_class(field.get_col(self.lhs.alias), self.rhs[column])
+            for lookup_class, field, column in zip(
+                lookup_classes, fields, self.lhs.field.columns
+            )
+        ]
+
+        value_constraint = WhereNode()
+        for lookup in lookups:
+            value_constraint.add(lookup, AND)
+        return value_constraint.as_sql(compiler, connection)