Add Deadline relationship to DagRun and update Deadline model (apache#50925)

* Add Deadline relationship to DagRun and update Deadline model

* Refactor Deadline relationship in DagRun and Deadline models

* add test

* call it deadlines

* remove test

Author: rawwar

airflow-core/src/airflow/models/dagrun.py

@@ -256,6 +256,13 @@ class DagRun(Base, LoggingMixin):
         cascade="all, delete, delete-orphan",
     )
 
+    deadlines = relationship(
+        "Deadline",
+        back_populates="dagrun",
+        uselist=True,
+        cascade="all, delete, delete-orphan",
+    )
+
     created_dag_version = relationship("DagVersion", uselist=False, passive_deletes=True)
     """
     The dag version that was active when the dag run was created, if available.

airflow-core/src/airflow/models/deadline.py

@@ -24,6 +24,7 @@
 import sqlalchemy_jsonfield
 import uuid6
 from sqlalchemy import Column, ForeignKey, Index, Integer, String
+from sqlalchemy.orm import relationship
 from sqlalchemy_utils import UUIDType
 
 from airflow.models.base import Base, StringID
@@ -56,6 +57,8 @@ class Deadline(Base):
     # Serialized kwargs to pass to the callback.
     callback_kwargs = Column(sqlalchemy_jsonfield.JSONField(json=json))
 
+    dagrun = relationship("DagRun", back_populates="deadlines")
+
     __table_args__ = (Index("deadline_idx", deadline, unique=False),)
 
     def __init__(