?_filter_column=col&_filter_op=isnull__1 redirect

if filter_op contains a __ the value is set to the right hand side.

e.g.

    ?_filter_column=col&_filter_op=isnull__1&_filter_value=x

Redirects to:

    ?col__isnull=1

Refs #86

datasette/app.py

@@ -431,6 +431,8 @@ class TableView(BaseView):
             filter_column = special_args['_filter_column']
             filter_op = special_args.get('_filter_op') or ''
             filter_value = special_args.get('_filter_value') or ''
+            if '__' in filter_op:
+                filter_op, filter_value = filter_op.split('__', 1)
             return self.redirect(request, path_with_added_args(request, {
                 '{}__{}'.format(filter_column, filter_op): filter_value,
                 '_filter_column': None,

tests/test_app.py

@@ -249,6 +249,16 @@ def test_add_filter_redirects(app_client):
     assert response.status == 302
     assert response.headers['Location'].endswith('?content__startswith=x&foo=bar')
 
+    # Test that op with a __x suffix overrides the filter value
+    path = path_base + '?' + urllib.parse.urlencode({
+        '_filter_column': 'content',
+        '_filter_op': 'isnull__5',
+        '_filter_value': 'x'
+    })
+    response = app_client.get(path, allow_redirects=False, gather_request=False)
+    assert response.status == 302
+    assert response.headers['Location'].endswith('?content__isnull=5')
+
 
 TABLES = '''
 CREATE TABLE simple_primary_key (