Add UserDomainRole table and helpers

src/registrar/models/user_domain_role.py

@@ -0,0 +1,49 @@
+from django.db import models
+
+from .utility.time_stamped_model import TimeStampedModel
+
+class UserDomainRole(TimeStampedModel):
+
+    """This is a linking table that connects a user with a role on a domain."""
+
+    class Roles(models.TextChoices):
+
+        """The possible roles are listed here.
+
+        Implementation of the named roles for allowing particular operations happens
+        elsewhere.
+        """
+
+        ADMIN = "admin"
+
+    user = models.ForeignKey(
+        "registrar.User",
+        null=False,
+        on_delete=models.CASCADE,  # when a user is deleted, their permissions will be too
+        related_name="permissions",
+    )
+
+    domain = models.ForeignKey(
+        "registrar.Domain",
+        null=False,
+        on_delete=models.CASCADE,  # when a domain is deleted, permissions are too
+        related_name="permissions"
+    )
+
+    role = models.TextField(
+        choices=Roles.choices,
+        null=False,
+        blank=False,
+    )
+
+    def __str__(self):
+        return "User {} is {} on domain {}".format(self.user, self.role, self.domain)
+
+    class Meta:
+        constraints = [
+            # a user can have only one role on a given domain, that is, there can
+            # be only a single row with a certain (user, domain) pair.
+            models.UniqueConstraint(
+                fields=['user', 'domain'], name='unique_user_domain_role'
+            )
+        ]