?_where= parameter on table views, closes #429

From pull request #430

datasette/static/app.css

@@ -105,6 +105,11 @@ h2 em {
     font-style: normal;
     font-weight: lighter;
 }
+.extra-wheres ul, .extra-wheres li {
+    list-style-type: none;
+    padding: 0;
+    margin: 0;
+}
 form.sql textarea {
     border: 1px solid #ccc;
     width: 70%;

datasette/templates/table.html

@@ -91,6 +91,17 @@
     </div>
 </form>
 
+{% if extra_wheres_for_ui %}
+<div class="extra-wheres">
+    <h3>{{ extra_wheres_for_ui|length }} extra where clause{% if extra_wheres_for_ui|length != 1 %}s{% endif %}</h3>
+    <ul>
+    {% for extra_where in extra_wheres_for_ui %}
+        <li><code>{{ extra_where.text }}</code> [<a href="{{ extra_where.remove_url }}">remove</a>]</li>
+    {% endfor %}
+    </ul>
+</div>
+{% endif %}
+
 {% if query.sql and config.allow_sql %}
     <p><a class="not-underlined" title="{{ query.sql }}" href="{{ database_url(database) }}?{{ {'sql': query.sql}|urlencode|safe }}{% if query.params %}&amp;{{ query.params|urlencode|safe }}{% endif %}">&#x270e; <span class="underlined">View and edit SQL</span></a></p>
 {% endif %}

datasette/views/table.py

@@ -295,6 +295,20 @@ class TableView(RowTableShared):
         filters = Filters(sorted(other_args.items()), units, ureg)
         where_clauses, params = filters.build_where_clauses(table)
 
+        extra_wheres_for_ui = []
+        # Add _where= from querystring
+        if "_where" in request.args:
+            if not self.ds.config("allow_sql"):
+                raise DatasetteError("_where= is not allowed", status=400)
+            else:
+                where_clauses.extend(request.args["_where"])
+                extra_wheres_for_ui = [{
+                    "text": text,
+                    "remove_url": path_with_removed_args(
+                        request, {"_where": text}
+                    )
+                } for text in request.args["_where"]]
+
         # _search support:
         fts_table = special_args.get("_fts_table")
         fts_table = fts_table or table_metadata.get("fts_table")
@@ -751,6 +765,7 @@ class TableView(RowTableShared):
                     key=lambda f: (len(f["results"]), f["name"]),
                     reverse=True
                 ),
+                "extra_wheres_for_ui": extra_wheres_for_ui,
                 "form_hidden_args": form_hidden_args,
                 "facet_hideable": lambda facet: facet not in metadata_facets,
                 "is_sortable": any(c["sortable"] for c in display_columns),