Unit test for register_facet_classes plugin, closes #773

I was a bit lazy with this one. I didn't hook up a test for the facet_results mechanism.
The custom facet hook isn't a great design so I will probably rethink it at some point
in the future anyway.

tests/plugins/my_plugin.py

@@ -1,4 +1,6 @@
 from datasette import hookimpl
+from datasette.facets import Facet
+from datasette.utils import path_with_added_args
 import base64
 import pint
 import json
@@ -92,3 +94,35 @@ def extra_template_vars(template, database, table, view_name, request, datasette
 @hookimpl
 def prepare_jinja2_environment(env):
     env.filters["format_numeric"] = lambda s: "{:,.0f}".format(float(s))
+
+
+@hookimpl
+def register_facet_classes():
+    return [DummyFacet]
+
+
+class DummyFacet(Facet):
+    type = "dummy"
+
+    async def suggest(self):
+        columns = await self.get_columns(self.sql, self.params)
+        return (
+            [
+                {
+                    "name": column,
+                    "toggle_url": self.ds.absolute_url(
+                        self.request,
+                        path_with_added_args(self.request, {"_facet_dummy": column}),
+                    ),
+                    "type": "dummy",
+                }
+                for column in columns
+            ]
+            if self.request.args.get("_dummy_facet")
+            else []
+        )
+
+    async def facet_results(self):
+        facet_results = {}
+        facets_timed_out = []
+        return facet_results, facets_timed_out

tests/test_plugins.py

@@ -428,3 +428,44 @@ def test_publish_subcommand():
     # advantage of the fact that cloudrun/heroku use the plugin hook
     # to register themselves as default plugins.
     assert ["cloudrun", "heroku"] == cli.publish.list_commands({})
+
+
+def test_register_facet_classes(app_client):
+    response = app_client.get(
+        "/fixtures/compound_three_primary_keys.json?_dummy_facet=1"
+    )
+    data = json.loads(response.body)
+    assert [
+        {
+            "name": "pk1",
+            "toggle_url": "http://localhost/fixtures/compound_three_primary_keys.json?_dummy_facet=1&_facet_dummy=pk1",
+            "type": "dummy",
+        },
+        {
+            "name": "pk2",
+            "toggle_url": "http://localhost/fixtures/compound_three_primary_keys.json?_dummy_facet=1&_facet_dummy=pk2",
+            "type": "dummy",
+        },
+        {
+            "name": "pk3",
+            "toggle_url": "http://localhost/fixtures/compound_three_primary_keys.json?_dummy_facet=1&_facet_dummy=pk3",
+            "type": "dummy",
+        },
+        {
+            "name": "content",
+            "toggle_url": "http://localhost/fixtures/compound_three_primary_keys.json?_dummy_facet=1&_facet_dummy=content",
+            "type": "dummy",
+        },
+        {
+            "name": "pk1",
+            "toggle_url": "http://localhost/fixtures/compound_three_primary_keys.json?_dummy_facet=1&_facet=pk1",
+        },
+        {
+            "name": "pk2",
+            "toggle_url": "http://localhost/fixtures/compound_three_primary_keys.json?_dummy_facet=1&_facet=pk2",
+        },
+        {
+            "name": "pk3",
+            "toggle_url": "http://localhost/fixtures/compound_three_primary_keys.json?_dummy_facet=1&_facet=pk3",
+        },
+    ] == data["suggested_facets"]