feat: add ExternalConfig.connection_id property to connect to exter…

…nal sources (#560)

* feat: add `ExternalConfig.connection_id` property to connect to external sources

* add tests

* fix unit tests

google/cloud/bigquery/external_config.py

@@ -760,6 +760,23 @@ def schema(self):
         prop = self._properties.get("schema", {})
         return [SchemaField.from_api_repr(field) for field in prop.get("fields", [])]
 
+    @property
+    def connection_id(self):
+        """Optional[str]: [Experimental] ID of a BigQuery Connection API
+        resource.
+
+        .. WARNING::
+
+           This feature is experimental. Pre-GA features may have limited
+           support, and changes to pre-GA features may not be compatible with
+           other pre-GA versions.
+        """
+        return self._properties.get("connectionId")
+
+    @connection_id.setter
+    def connection_id(self, value):
+        self._properties["connectionId"] = value
+
     @schema.setter
     def schema(self, value):
         prop = value

tests/unit/test_external_config.py

@@ -74,6 +74,7 @@ def test_to_api_repr_base(self):
         ec.autodetect = True
         ec.ignore_unknown_values = False
         ec.compression = "compression"
+        ec.connection_id = "path/to/connection"
         ec.schema = [schema.SchemaField("full_name", "STRING", mode="REQUIRED")]
 
         exp_schema = {
@@ -87,10 +88,17 @@ def test_to_api_repr_base(self):
             "autodetect": True,
             "ignoreUnknownValues": False,
             "compression": "compression",
+            "connectionId": "path/to/connection",
             "schema": exp_schema,
         }
         self.assertEqual(got_resource, exp_resource)
 
+    def test_connection_id(self):
+        ec = external_config.ExternalConfig("")
+        self.assertIsNone(ec.connection_id)
+        ec.connection_id = "path/to/connection"
+        self.assertEqual(ec.connection_id, "path/to/connection")
+
     def test_schema_None(self):
         ec = external_config.ExternalConfig("")
         ec.schema = None