fix: Display a better error message when database URI is null (#8015)

src/meltano/core/db.py

@@ -37,6 +37,21 @@ def __init__(self, reason: str):
         super().__init__(reason, self.INSTRUCTION)
 
 
+class NullConnectionStringError(MeltanoError):
+    """Raised when the database is not compatible with Meltano."""
+
+    REASON = "The `database_uri` setting has a null value"
+    INSTRUCTION = (
+        "Verify that the `database_uri` setting points to a valid database connection "
+        "URI, or use `MELTANO_FF_STRICT_ENV_VAR_MODE=1 meltano config meltano list` "
+        "to check for missing environment variables"
+    )
+
+    def __init__(self):
+        """Initialize the exception."""
+        super().__init__(self.REASON, self.INSTRUCTION)
+
+
 def project_engine(
     project: Project,
     default: bool = False,
@@ -50,6 +65,9 @@ def project_engine(
 
     Returns:
         The engine, and a session maker bound to the engine.
+
+    Raises:
+        NullConnectionStringError: The `database_uri` setting has a null value.
     """
     existing_engine = _engines.get(project)
     if existing_engine:
@@ -58,6 +76,9 @@ def project_engine(
     engine_uri = project.settings.get("database_uri")
     logging.debug(f"Creating engine '{project}@{engine_uri}'")
 
+    if engine_uri is None:
+        raise NullConnectionStringError
+
     engine = create_engine(engine_uri, poolclass=NullPool)
 
     # Connect to the database to ensure it is available.

tests/meltano/core/test_db_reconnection.py → tests/meltano/core/test_db_connection.py

@@ -1,10 +1,17 @@
 from __future__ import annotations
 
+import typing as t
+
 import pytest
+import yaml
 from mock import Mock
 from sqlalchemy.exc import OperationalError
 
-from meltano.core.db import connect
+from meltano.core.db import NullConnectionStringError, connect, project_engine
+from meltano.core.project import Project
+
+if t.TYPE_CHECKING:
+    from pathlib import Path
 
 
 class TestConnectionRetries:
@@ -31,3 +38,25 @@ def test_ping_failure(self):
 
         connect(engine=engine_mock, max_retries=3, retry_timeout=0.1)
         assert engine_mock.connect.call_count == 2
+
+
+class TestProjectEngine:
+    def test_project_engine(
+        self,
+        monkeypatch: pytest.MonkeyPatch,
+        tmp_path: Path,
+    ):
+        with tmp_path.joinpath("meltano.yml").open("w") as meltano_yml:
+            yaml.dump(
+                {
+                    "project_id": "test",
+                    "send_anonymous_usage_stat": False,
+                    "database_uri": "$DATABASE",
+                },
+                meltano_yml,
+            )
+        monkeypatch.delenv("MELTANO_DATABASE_URI")
+
+        project = Project(tmp_path)
+        with pytest.raises(NullConnectionStringError):
+            project_engine(project)