Adding metric expression validation

.changes/unreleased/Under the Hood-20220916-154712.yaml

@@ -0,0 +1,7 @@
+kind: Under the Hood
+body: Adding validation for metric expression attribute
+time: 2022-09-16T15:47:12.799002-05:00
+custom:
+  Author: callum-mcdata
+  Issue: "5871"
+  PR: "5873"

core/dbt/contracts/graph/parsed.py

@@ -819,8 +819,8 @@ class ParsedMetric(UnparsedBaseNode, HasUniqueID, HasFqn):
     description: str
     label: str
     calculation_method: str
-    expression: str
     timestamp: str
+    expression: str
     filters: List[MetricFilter]
     time_grains: List[str]
     dimensions: List[str]

core/dbt/contracts/graph/unparsed.py

@@ -475,8 +475,8 @@ class UnparsedMetric(dbtClassMixin, Replaceable):
     label: str
     calculation_method: str
     timestamp: str
+    expression: str
     description: str = ""
-    expression: Union[str, int] = ""
     time_grains: List[str] = field(default_factory=list)
     dimensions: List[str] = field(default_factory=list)
     window: Optional[MetricTime] = None

tests/functional/metrics/test_metrics.py

@@ -196,6 +196,42 @@ def test_simple_metric(
             run_dbt(["run"])
 
 
+invalid_metrics__missing_expression_yml = """
+version: 2
+metrics:
+  - name: number_of_people
+    label: "Number of people"
+    model: "ref(people)"
+    description: Total count of people
+    calculation_method: count
+    timestamp: created_at
+    time_grains: [day, week, month]
+    dimensions:
+      - favorite_color
+      - loves_dbt
+    meta:
+        my_meta: 'testing'
+"""
+
+
+class TestInvalidMetricMissingExpression:
+    @pytest.fixture(scope="class")
+    def models(self):
+        return {
+            "people_metrics.yml": invalid_metrics__missing_expression_yml,
+            "people.sql": models__people_sql,
+        }
+
+    # tests that we get a ParsingException with a missing expression
+    def test_simple_metric(
+        self,
+        project,
+    ):
+        # initial run
+        with pytest.raises(ParsingException):
+            run_dbt(["run"])
+
+
 names_with_spaces_metrics_yml = """
 version: 2