diff --git a/execution_engine/execution_engine.py b/execution_engine/execution_engine.py
index 6f505aee..bb591600 100644
--- a/execution_engine/execution_engine.py
+++ b/execution_engine/execution_engine.py
@@ -118,7 +118,7 @@ def execute(
         """
         Executes a recommendation and stores the results in the result database.
 
-        :param recommendation: The Recommendation object (loaded from ExectionEngine.load_recommendation).
+        :param recommendation: The Recommendation object (loaded from ExecutionEngine.load_recommendation).
         :param start_datetime: The start of the observation window.
         :param end_datetime: The end of the observation window. If None, the current time is used.
         :return: The ID of the run.
@@ -135,7 +135,12 @@ def execute(
         )
 
         with self._db.begin():
-            self.register_recommendation(recommendation)
+            # If the recommendation has been loaded from the
+            # database, its _id slot is not None. Otherwise, register
+            # the recommendation to store it into the database and
+            # assign an id.
+            if recommendation._id is None:
+                self.register_recommendation(recommendation)
             run_id = self.register_run(
                 recommendation, start_datetime=start_datetime, end_datetime=end_datetime
             )
@@ -186,7 +191,14 @@ def load_recommendation_from_database(
             recommendation = cohort.Recommendation.from_json(
                 rec_db.recommendation_json.decode()
             )
-            recommendation.id = rec_db.recommendation_id
+            # All objects in the deserialized object graph must have
+            # an id.
+            assert recommendation._id is not None
+            assert recommendation._base_criterion._id is not None
+            for pi_pair in recommendation._pi_pairs:
+                assert pi_pair._id is not None
+                for criterion in pi_pair.flatten():
+                    assert criterion._id is not None
             return recommendation
 
         return None
@@ -198,14 +210,29 @@ def _hash(obj: Serializable) -> tuple[bytes, str]:
 
     def register_recommendation(self, recommendation: cohort.Recommendation) -> None:
         """Registers the Recommendation in the result database."""
-
+        # We don't want to include any ids in the hash since ids
+        # "accidental" in the sense that they depend on, at least, the
+        # order in which recommendations are inserted into the
+        # database.
+        assert recommendation._id is None
+        assert recommendation._base_criterion._id is None
+        for pi_pair in recommendation._pi_pairs:
+            assert pi_pair._id is None
+            for criterion in pi_pair.flatten():
+                assert criterion._id is None
+        # Get the hash but ignore the JSON representation for now
+        # since we will compute and insert a complete JSON
+        # representation later when we know all ids.
+        _, rec_hash = self._hash(recommendation)
         recommendation_table = result_db.Recommendation
-
-        rec_json, rec_hash = self._hash(recommendation)
+        # We look for a recommendation with the computed hash in the
+        # database. If there is one, set the id of our recommendation
+        # to the stored id. Otherwise, store our recommendation
+        # (without the JSON representation) in the database and
+        # receive the fresh id.
         query = select(recommendation_table).where(
             recommendation_table.recommendation_hash == rec_hash
         )
-
         with self._db.begin() as con:
             rec_db = con.execute(query).fetchone()
 
@@ -221,7 +248,7 @@ def register_recommendation(self, recommendation: cohort.Recommendation) -> None
                         recommendation_version=recommendation.version,
                         recommendation_package_version=recommendation.package_version,
                         recommendation_hash=rec_hash,
-                        recommendation_json=rec_json,
+                        recommendation_json=bytes(),  # updated later
                         create_datetime=datetime.now(),
                     )
                     .returning(recommendation_table.recommendation_id)
@@ -229,25 +256,43 @@ def register_recommendation(self, recommendation: cohort.Recommendation) -> None
 
                 result = con.execute(query)
                 recommendation.id = result.fetchone().recommendation_id
-
+        # Register all child objects. After that, the recommendation
+        # and all child objects have valid ids (either restored or
+        # fresh).
+        self.register_criterion(recommendation._base_criterion)
         for pi_pair in recommendation.population_intervention_pairs():
             self.register_population_intervention_pair(
                 pi_pair, recommendation_id=recommendation.id
             )
-
             for criterion in pi_pair.flatten():
                 self.register_criterion(criterion)
 
+        assert recommendation.id is not None
+        # TODO(jmoringe): mypy doesn't like this one. Not sure why.
+        # assert recommendation._base_criterion._id is not None
+        for pi_pair in recommendation._pi_pairs:
+            assert pi_pair._id is not None
+            for criterion in pi_pair.flatten():
+                assert criterion._id is not None
+
+        # Update the recommendation in the database with the final
+        # JSON representation and execution graph (now that
+        # recommendation id, criteria ids and pi pair is are known)
+        # TODO(jmoringe): only when necessary
         with self._db.begin() as con:
-            # update recommendation with execution graph (now that criterion & pi pair is are known)
             rec_graph: bytes = json.dumps(
                 recommendation.execution_graph().to_cytoscape_dict(), sort_keys=True
             ).encode()
 
+            rec_json: bytes = recommendation.json()
+            logging.info(f"Storing recommendation {recommendation}")
             update_query = (
                 update(recommendation_table)
                 .where(recommendation_table.recommendation_id == recommendation.id)
-                .values(recommendation_execution_graph=rec_graph)
+                .values(
+                    recommendation_json=rec_json,
+                    recommendation_execution_graph=rec_graph,
+                )
             )
 
             con.execute(update_query)
@@ -261,6 +306,8 @@ def register_population_intervention_pair(
         :param pi_pair: The Population/Intervention Pair.
         :param recommendation_id: The ID of the Population/Intervention Pair.
         """
+        # We don't want to include the id in the hash
+        assert pi_pair._id is None
         _, pi_pair_hash = self._hash(pi_pair)
         query = select(result_db.PopulationInterventionPair).where(
             result_db.PopulationInterventionPair.pi_pair_hash == pi_pair_hash
@@ -314,6 +361,8 @@ def register_criterion(self, criterion: Criterion) -> None:
                 result = con.execute(query)
                 criterion.id = result.fetchone().criterion_id
 
+        assert criterion.id is not None
+
     def register_run(
         self,
         recommendation: cohort.Recommendation,
diff --git a/execution_engine/omop/cohort/population_intervention_pair.py b/execution_engine/omop/cohort/population_intervention_pair.py
index 5e3d7343..ab7100dd 100644
--- a/execution_engine/omop/cohort/population_intervention_pair.py
+++ b/execution_engine/omop/cohort/population_intervention_pair.py
@@ -38,7 +38,7 @@ class PopulationInterventionPair(Serializable):
     (e.g. "has condition X and lab value Y >= Z").
     """
 
-    _id: int | None = None
+    _id: int | None
     _name: str
     _population: CriterionCombination
     _intervention: CriterionCombination
@@ -50,7 +50,9 @@ def __init__(
         base_criterion: Criterion,
         population: CriterionCombination | None = None,
         intervention: CriterionCombination | None = None,
+        id: int | None = None,
     ) -> None:
+        self._id = id
         self._name = name
         self._url = url
         self._base_criterion = base_criterion
@@ -231,6 +233,7 @@ def dict(self) -> dict[str, Any]:
         population = self._population
         intervention = self._intervention
         return {
+            "id": self._id,
             "name": self.name,
             "url": self.url,
             "base_criterion": {
@@ -262,12 +265,13 @@ def from_dict(cls, data: Dict[str, Any]) -> "PopulationInterventionPair":
             CriterionCombination, criterion_factory(**data["intervention"])
         )
         object = cls(
+            id=data["id"],
             name=data["name"],
             url=data["url"],
             base_criterion=base_criterion,
         )
         # The constructor initializes the population and intervention
-        # slots in a particular way but we want to use whatever we
+        # slots in a particular way, but we want to use whatever we
         # have deserialized instead. This is a bit inefficient because
         # we discard the values that were assigned to the two slots in
         # the constructor.
diff --git a/execution_engine/omop/cohort/recommendation.py b/execution_engine/omop/cohort/recommendation.py
index a6c7a45a..120fd13e 100644
--- a/execution_engine/omop/cohort/recommendation.py
+++ b/execution_engine/omop/cohort/recommendation.py
@@ -249,10 +249,14 @@ def dict(self) -> dict:
         """
         Get the combination as a dictionary.
         """
+        base_criterion = self._base_criterion
         return {
             "id": self._id,
             "population_intervention_pairs": [c.dict() for c in self._pi_pairs],
-            "base_criterion": self._base_criterion.dict(),
+            "base_criterion": {
+                "class_name": base_criterion.__class__.__name__,
+                "data": base_criterion.dict(),
+            },
             "recommendation_name": self._name,
             "recommendation_title": self._title,
             "recommendation_url": self._url,
@@ -283,5 +287,5 @@ def from_dict(cls, data: Dict[str, Any]) -> Self:
             version=data["recommendation_version"],
             description=data["recommendation_description"],
             package_version=data["recommendation_package_version"],
-            recommendation_id=data["id"] if "id" in data else None,
+            recommendation_id=data["id"],
         )
diff --git a/execution_engine/omop/criterion/abstract.py b/execution_engine/omop/criterion/abstract.py
index d80f58b4..0d4cf0c4 100644
--- a/execution_engine/omop/criterion/abstract.py
+++ b/execution_engine/omop/criterion/abstract.py
@@ -93,8 +93,10 @@ class AbstractCriterion(Serializable, ABC):
     Abstract base class for Criterion and CriterionCombination.
     """
 
-    def __init__(self, category: CohortCategory) -> None:
-        self._id = None
+    def __init__(
+        self, category: CohortCategory, id: int | None = None
+    ) -> None:
+        self._id = id
 
         assert isinstance(
             category, CohortCategory
@@ -210,8 +212,10 @@ class Criterion(AbstractCriterion):
     Flag to indicate whether the filter_datetime function has been called.
     """
 
-    def __init__(self, category: CohortCategory) -> None:
-        super().__init__(category=category)
+    def __init__(
+        self, category: CohortCategory, id: int | None = None
+    ) -> None:
+        super().__init__(category=category, id=id)
 
     def _set_omop_variables_from_domain(self, domain_id: str) -> None:
         """
diff --git a/execution_engine/omop/criterion/combination/combination.py b/execution_engine/omop/criterion/combination/combination.py
index b671584a..79740e65 100644
--- a/execution_engine/omop/criterion/combination/combination.py
+++ b/execution_engine/omop/criterion/combination/combination.py
@@ -143,9 +143,13 @@ def dict(self) -> dict[str, Any]:
             "threshold": self._operator.threshold,
             "category": self._category.value,
             "criteria": [
-                {"class_name": criterion.__class__.__name__, "data": criterion.dict()}
+                {
+                    "class_name": criterion.__class__.__name__,
+                    "data": criterion.dict(),
+                }
                 for criterion in self._criteria
             ],
+            "root": self._root,
         }
 
     def __invert__(self) -> AbstractCriterion:
@@ -192,6 +196,7 @@ def from_dict(cls, data: Dict[str, Any]) -> "CriterionCombination":
         combination = cls(
             operator=operator,
             category=category,
+            root_combination=data["root"],
         )
 
         for criterion in data["criteria"]:
diff --git a/execution_engine/omop/criterion/concept.py b/execution_engine/omop/criterion/concept.py
index 838474dc..2474db4d 100644
--- a/execution_engine/omop/criterion/concept.py
+++ b/execution_engine/omop/criterion/concept.py
@@ -43,12 +43,13 @@ def __init__(
         self,
         category: CohortCategory,
         concept: Concept,
+        id: int | None = None,
         value: Value | None = None,
         static: bool | None = None,
         timing: Timing | None = None,
         override_value_required: bool | None = None,
     ):
-        super().__init__(category=category)
+        super().__init__(category=category, id=id)
 
         self._set_omop_variables_from_domain(concept.domain_id)
         self._concept = concept
@@ -135,6 +136,7 @@ def dict(self) -> dict[str, Any]:
         Get a JSON representation of the criterion.
         """
         return {
+            "id": self._id,
             "category": self._category.value,
             "concept": self._concept.model_dump(),
             "value": (
@@ -157,6 +159,7 @@ def from_dict(cls, data: Dict[str, Any]) -> "ConceptCriterion":
         """
 
         return cls(
+            id=data["id"],
             category=CohortCategory(data["category"]),
             concept=Concept(**data["concept"]),
             value=(
diff --git a/execution_engine/omop/criterion/drug_exposure.py b/execution_engine/omop/criterion/drug_exposure.py
index e51c8a5c..762e6ffa 100644
--- a/execution_engine/omop/criterion/drug_exposure.py
+++ b/execution_engine/omop/criterion/drug_exposure.py
@@ -33,11 +33,12 @@ def __init__(
         ingredient_concept: Concept,
         dose: Dosage | None,
         route: Concept | None,
+        id: int | None = None,
     ) -> None:
         """
         Initialize the drug administration action.
         """
-        super().__init__(category=category)
+        super().__init__(category=category, id=id)
         self._set_omop_variables_from_domain("drug")
         self._ingredient_concept = ingredient_concept
 
@@ -356,6 +357,7 @@ def dict(self) -> dict[str, Any]:
         Return a dictionary representation of the criterion.
         """
         return {
+            "id": self._id,
             "category": self._category.value,
             "ingredient_concept": self._ingredient_concept.model_dump(),
             "dose": (
@@ -377,6 +379,7 @@ def from_dict(cls, data: Dict[str, Any]) -> "DrugExposure":
         assert dose is None or isinstance(dose, Dosage), "Dose must be a Dosage or None"
 
         return cls(
+            id=data["id"],
             category=CohortCategory(data["category"]),
             ingredient_concept=Concept(**data["ingredient_concept"]),
             dose=dose,
diff --git a/execution_engine/omop/criterion/procedure_occurrence.py b/execution_engine/omop/criterion/procedure_occurrence.py
index 9355f951..58f2adb8 100644
--- a/execution_engine/omop/criterion/procedure_occurrence.py
+++ b/execution_engine/omop/criterion/procedure_occurrence.py
@@ -29,12 +29,14 @@ def __init__(
         value: ValueNumber | None = None,
         timing: Timing | None = None,
         static: bool | None = None,
+        id: int | None = None,
     ) -> None:
         super().__init__(
             category=category,
             concept=concept,
             value=value,
             static=static,
+            id=id,
         )
 
         self._set_omop_variables_from_domain("procedure")
@@ -158,6 +160,7 @@ def dict(self) -> dict[str, Any]:
         assert self._concept is not None, "Concept must be set"
 
         return {
+            "id": self._id,
             "category": self._category.value,
             "concept": self._concept.model_dump(),
             "value": (
@@ -189,6 +192,7 @@ def from_dict(cls, data: Dict[str, Any]) -> "ProcedureOccurrence":
         ), "timing must be a ValueNumber"
 
         return cls(
+            id=data["id"],
             category=CohortCategory(data["category"]),
             concept=Concept(**data["concept"]),
             value=value,
diff --git a/execution_engine/omop/criterion/visit_occurrence.py b/execution_engine/omop/criterion/visit_occurrence.py
index d28180d6..fd9a9cfe 100644
--- a/execution_engine/omop/criterion/visit_occurrence.py
+++ b/execution_engine/omop/criterion/visit_occurrence.py
@@ -23,8 +23,10 @@ class ActivePatients(VisitOccurrence):
     Select only patients who are still hospitalized.
     """
 
-    def __init__(self) -> None:
+    def __init__(self, id: int | None = None) -> None:
+        # TODO(jmoringe): why not use the constructor? super().__init__(id=id)
         self._category = CohortCategory.BASE
+        self._id = id
 
         if get_config().episode_of_care_visit_detail:
             self._set_omop_variables_from_domain("visit_detail")
@@ -86,14 +88,14 @@ def dict(self) -> dict[str, Any]:
         """
         Get a JSON representation of the criterion.
         """
-        return {"class_name": self.__class__.__name__, "data": {}}
+        return {"id": self._id}
 
     @classmethod
     def from_dict(cls, data: Dict[str, Any]) -> "ActivePatients":
         """
         Create a criterion from a JSON representation.
         """
-        return cls()
+        return cls(id=data["id"])
 
 
 class PatientsActiveDuringPeriod(ActivePatients):
diff --git a/execution_engine/omop/serializable.py b/execution_engine/omop/serializable.py
index 703c135c..a7d4f4fb 100644
--- a/execution_engine/omop/serializable.py
+++ b/execution_engine/omop/serializable.py
@@ -51,13 +51,10 @@ def json(self) -> bytes:
 
         s_json = self.dict()
 
-        if "id" in s_json:
-            del s_json["id"]
-
         return json.dumps(s_json, sort_keys=True).encode()
 
     @classmethod
-    def from_json(cls, data: str) -> Self:
+    def from_json(cls, data: str | bytes) -> Self:
         """
         Create a combination from a JSON string.
         """
diff --git a/tests/execution_engine/omop/cohort/test_cohort_recommendation.py b/tests/execution_engine/omop/cohort/test_cohort_recommendation.py
new file mode 100644
index 00000000..87defac8
--- /dev/null
+++ b/tests/execution_engine/omop/cohort/test_cohort_recommendation.py
@@ -0,0 +1,60 @@
+import pytest
+
+from execution_engine.omop.cohort.recommendation import Recommendation
+from execution_engine.omop.concepts import Concept
+from execution_engine.omop.criterion.visit_occurrence import ActivePatients
+from tests.mocks.criterion import MockCriterion
+
+
+class TestRecommendation:
+
+    def test_serialization(self):
+        # Register the mock criterion class
+        from execution_engine.omop.criterion import factory
+
+        factory.register_criterion_class("MockCriterion", MockCriterion)
+
+        original = Recommendation(
+            pi_pairs=[],
+            base_criterion=MockCriterion("c"),
+            name="foo",
+            title="bar",
+            url="baz",
+            version="1.0",
+            description="hi",
+        )
+
+        json = original.json()
+        deserialized = Recommendation.from_json(json)
+        assert original == deserialized
+
+    @pytest.fixture
+    def concept(self):
+        return Concept(
+            concept_id=32037,
+            concept_name="Intensive Care",
+            domain_id="Visit",
+            vocabulary_id="Visit",
+            concept_class_id="Visit",
+            standard_concept="S",
+            concept_code="OMOP4822460",
+            invalid_reason=None,
+        )
+
+    def test_serialization_with_active_patients(self, concept):
+        # Test with a ActivePatients as the base criterion
+        # specifically because there used to be a problem with the
+        # serialization of that combination.
+        original = Recommendation(
+            pi_pairs=[],
+            base_criterion=ActivePatients(),
+            name="foo",
+            title="bar",
+            url="baz",
+            version="1.0",
+            description="hi",
+        )
+
+        json = original.json()
+        deserialized = Recommendation.from_json(json)
+        assert original == deserialized
diff --git a/tests/execution_engine/omop/criterion/combination/test_logical_combination.py b/tests/execution_engine/omop/criterion/combination/test_logical_combination.py
index 0a8ba06a..e088a603 100644
--- a/tests/execution_engine/omop/criterion/combination/test_logical_combination.py
+++ b/tests/execution_engine/omop/criterion/combination/test_logical_combination.py
@@ -109,6 +109,7 @@ def test_criterion_combination_dict(self, mock_criteria):
                 {"class_name": "MockCriterion", "data": criterion.dict()}
                 for criterion in mock_criteria
             ],
+            "root": False,
         }
 
     def test_criterion_combination_from_dict(self, mock_criteria):
@@ -123,6 +124,7 @@ def test_criterion_combination_from_dict(self, mock_criteria):
                 {"class_name": "MockCriterion", "data": criterion.dict()}
                 for criterion in mock_criteria
             ],
+            "root": False,
         }
 
         # Register the mock criterion class
diff --git a/tests/execution_engine/omop/criterion/combination/test_temporal_combination.py b/tests/execution_engine/omop/criterion/combination/test_temporal_combination.py
index e4e58c49..cae1e71e 100644
--- a/tests/execution_engine/omop/criterion/combination/test_temporal_combination.py
+++ b/tests/execution_engine/omop/criterion/combination/test_temporal_combination.py
@@ -115,6 +115,7 @@ def test_criterion_combination_dict(self, mock_criteria):
                 {"class_name": "MockCriterion", "data": criterion.dict()}
                 for criterion in mock_criteria
             ],
+            "root": False,
         }
 
     def test_criterion_combination_from_dict(self, mock_criteria):
@@ -128,6 +129,7 @@ def test_criterion_combination_from_dict(self, mock_criteria):
         )
 
         combination_data = {
+            "id": None,
             "operator": "AT_LEAST",
             "threshold": 1,
             "category": "POPULATION_INTERVENTION",