Don't re-retrieve backend in session cm (#2282)

* don't reretrieve backend in session cm

* add release note

* Update release-notes/unreleased/2282.bug.rst

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

* add test

---------

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

Author: kt474

qiskit_ibm_runtime/base_primitive.py

@@ -86,19 +86,10 @@ def _get_mode_service_backend(mode: Optional[Union[BackendV2, Session, Batch]] =
         raise ValueError("mode must be of type Backend, Session, Batch or None")
     elif get_cm_session():
         mode = get_cm_session()
-        service = mode.service  # type: ignore
-        try:
-            backend = service.backend(
-                name=mode.backend(),  # type: ignore
-                instance=mode._instance,  # type: ignore
-                use_fractional_gates=mode._backend.options.use_fractional_gates,  # type: ignore
-            )
-        except (AttributeError, TypeError):
-            backend = service.backend(
-                name=mode.backend(),  # type: ignore
-                instance=mode._instance,  # type: ignore
-            )
-        return mode, service, backend  # type: ignore
+        service = mode.service
+        backend = mode._backend
+
+        return mode, service, backend
     else:
         raise ValueError("A backend or session must be specified.")
 

release-notes/unreleased/2282.bug.rst

@@ -0,0 +1,6 @@
+Fixed a bug in :class:`.BasePrimitive` where primitives instantiated inside a :class:`~.Session` or :class:`~.Batch` context manager without the ``mode`` 
+parameter would fetch the backend from the service (by name) instead of using the backend passed to the :class:`~.Session` or :class:`~.Batch`. 
+This could cause issues when the :class:`~.Session`/:class:`~.Batch`
+backend was modified by users (for example, by removing a gate), because the primitives 
+would instead fetch the unmodified backend object from the service. After the fix, the
+:class:`~.Session`/:class:`~.Batch` backend object is used directly. 

test/integration/test_session.py

@@ -17,6 +17,7 @@
 from qiskit.circuit.library import RealAmplitudes
 from qiskit.quantum_info import SparsePauliOp
 
+from qiskit.circuit import IfElseOp
 from qiskit.primitives import PrimitiveResult
 from qiskit.transpiler.preset_passmanagers import generate_preset_pass_manager
 
@@ -98,3 +99,21 @@ def test_session_from_id_no_backend(self, service):
         if session.details().get("backend_name") == "":
             with self.assertRaises(IBMRuntimeError):
                 Session.from_id(session_id=session._session_id, service=service)
+
+    @run_integration_test
+    def test_session_backend(self, service):
+        """Test session backend is the correct backend."""
+        backend = service.backend(self.dependencies.qpu)
+
+        pm = generate_preset_pass_manager(optimization_level=1, target=backend.target)
+        instruction_name = "test_name"
+        backend.target.add_instruction(IfElseOp, name=instruction_name)
+
+        with Session(backend=backend) as session:
+            sampler = SamplerV2(mode=session)
+            job = sampler.run([pm.run(bell())])
+            self.assertIn(instruction_name, job.backend().target.operation_names)
+
+            sampler2 = SamplerV2()
+            job2 = sampler2.run([pm.run(bell())])
+            self.assertIn(instruction_name, job2.backend().target.operation_names)