basic invitation model with domain class

atst/models/__init__.py

@@ -18,3 +18,4 @@ from .request_revision import RequestRevision
 from .request_review import RequestReview
 from .request_internal_comment import RequestInternalComment
 from .audit_event import AuditEvent
+from .invitation import Invitation

atst/models/invitation.py

@@ -0,0 +1,25 @@
+from sqlalchemy import Column, ForeignKey, Boolean
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import relationship
+
+from atst.models import Base, types
+from atst.models.mixins.timestamps import TimestampsMixin
+
+
+class Invitation(Base, TimestampsMixin):
+    __tablename__ = "invitations"
+
+    id = types.Id()
+
+    user_id = Column(UUID(as_uuid=True), ForeignKey("users.id"), index=True)
+    user = relationship("User", backref="invitations")
+
+    workspace_id = Column(UUID(as_uuid=True), ForeignKey("workspaces.id"), index=True)
+    workspace = relationship("Workspace", backref="invitations")
+
+    valid = Column(Boolean, default=True)
+
+    def __repr__(self):
+        return "<Invitation(user='{}', workspace='{}', id='{}')>".format(
+            self.user.id, self.workspace.id, self.id
+        )