Column metadata, closes #942

datasette/static/app.css

@@ -784,9 +784,14 @@ svg.dropdown-menu-icon {
     font-size: 0.7em;
     color: #666;
     margin: 0;
-    padding: 0;
     padding: 4px 8px 4px 8px;
 }
+.dropdown-menu .dropdown-column-description {
+    margin: 0;
+    color: #666;
+    padding: 4px 8px 4px 8px;
+    max-width: 20em;
+}
 .dropdown-menu li {
     border-bottom: 1px solid #ccc;
 }
@@ -836,6 +841,16 @@ svg.dropdown-menu-icon {
     background-repeat: no-repeat;
 }
 
+dl.column-descriptions dt {
+    font-weight: bold;
+}
+dl.column-descriptions dd {
+    padding-left: 1.5em;
+    white-space: pre-wrap;
+    line-height: 1.1em;
+    color: #666;
+}
+
 .anim-scale-in {
     animation-name: scale-in;
     animation-duration: 0.15s;

datasette/static/table.js

@@ -9,6 +9,7 @@ var DROPDOWN_HTML = `<div class="dropdown-menu">
   <li><a class="dropdown-not-blank" href="#">Show not-blank rows</a></li>
 </ul>
 <p class="dropdown-column-type"></p>
+<p class="dropdown-column-description"></p>
 </div>`;
 
 var DROPDOWN_ICON_SVG = `<svg xmlns="http://www.w3.org/2000/svg" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
@@ -166,6 +167,14 @@ var DROPDOWN_ICON_SVG = `<svg xmlns="http://www.w3.org/2000/svg" width="14" heig
     } else {
       columnTypeP.style.display = "none";
     }
+
+    var columnDescriptionP = menu.querySelector(".dropdown-column-description");
+    if (th.dataset.columnDescription) {
+      columnDescriptionP.innerText = th.dataset.columnDescription;
+      columnDescriptionP.style.display = "block";
+    } else {
+      columnDescriptionP.style.display = "none";
+    }
     menu.style.position = "absolute";
     menu.style.top = menuTop + 6 + "px";
     menu.style.left = menuLeft + "px";

datasette/templates/_table.html

@@ -4,7 +4,7 @@
         <thead>
             <tr>
                 {% for column in display_columns %}
-                    <th class="col-{{ column.name|to_css_class }}" scope="col" data-column="{{ column.name }}" data-column-type="{{ column.type }}" data-column-not-null="{{ column.notnull }}" data-is-pk="{% if column.is_pk %}1{% else %}0{% endif %}">
+                    <th {% if column.description %}data-column-description="{{ column.description }}" {% endif %}class="col-{{ column.name|to_css_class }}" scope="col" data-column="{{ column.name }}" data-column-type="{{ column.type }}" data-column-not-null="{{ column.notnull }}" data-is-pk="{% if column.is_pk %}1{% else %}0{% endif %}">
                         {% if not column.sortable %}
                             {{ column.name }}
                         {% else %}

datasette/templates/table.html

@@ -51,6 +51,14 @@
 
 {% block description_source_license %}{% include "_description_source_license.html" %}{% endblock %}
 
+{% if metadata.columns %}
+<dl class="column-descriptions">
+    {% for column_name, column_description in metadata.columns.items() %}
+        <dt>{{ column_name }}</dt><dd>{{ column_description }}</dd>
+    {% endfor %}
+</dl>
+{% endif %}
+
 {% if filtered_table_rows_count or human_description_en %}
     <h3>{% if filtered_table_rows_count or filtered_table_rows_count == 0 %}{{ "{:,}".format(filtered_table_rows_count) }} row{% if filtered_table_rows_count == 1 %}{% else %}s{% endif %}{% endif %}
         {% if human_description_en %}{{ human_description_en }}{% endif %}

datasette/views/table.py

@@ -125,6 +125,7 @@ class RowTableShared(DataView):
         """Returns columns, rows for specified table - including fancy foreign key treatment"""
         db = self.ds.databases[database]
         table_metadata = self.ds.table_metadata(database, table)
+        column_descriptions = table_metadata.get("columns") or {}
         column_details = {col.name: col for col in await db.table_column_details(table)}
         sortable_columns = await self.sortable_columns_for_table(database, table, True)
         pks = await db.primary_keys(table)
@@ -147,6 +148,7 @@ class RowTableShared(DataView):
                     "is_pk": r[0] in pks_for_display,
                     "type": type_,
                     "notnull": notnull,
+                    "description": column_descriptions.get(r[0]),
                 }
             )
 

docs/metadata.rst

@@ -78,6 +78,34 @@ The three visible metadata fields you can apply to everything, specific database
 
 For each of these you can provide just the ``*_url`` field and Datasette will treat that as the default link label text and display the URL directly on the page.
 
+.. _metadata_column_descriptions:
+
+Column descriptions
+-------------------
+
+You can include descriptions for your columns by adding a ``"columns": {"name-of-column": "description-of-column"}`` block to your table metadata:
+
+.. code-block:: json
+
+    {
+        "databases": {
+            "database1": {
+                "tables": {
+                    "example_table": {
+                        "columns": {
+                            "column1": "Description of column 1",
+                            "column2": "Description of column 2"
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+These will be displayed at the top of the table page, and will also show in the cog menu for each column.
+
+You can see an example of how these look at `latest.datasette.io/fixtures/roadside_attractions <https://latest.datasette.io/fixtures/roadside_attractions>`__.
+
 Specifying units for a column
 -----------------------------
 

tests/fixtures.py

@@ -336,6 +336,12 @@ METADATA = {
                     "fts_table": "searchable_fts",
                     "fts_pk": "pk",
                 },
+                "roadside_attractions": {
+                    "columns": {
+                        "name": "The name of the attraction",
+                        "address": "The street address for the attraction",
+                    }
+                },
                 "attraction_characteristic": {"sort_desc": "pk"},
                 "facet_cities": {"sort": "name"},
                 "paginated_view": {"size": 25},

tests/test_html.py

@@ -1777,3 +1777,21 @@ def test_trace_correctly_escaped(app_client):
     response = app_client.get("/fixtures?sql=select+'<h1>Hello'&_trace=1")
     assert "select '<h1>Hello" not in response.text
     assert "select &#39;&lt;h1&gt;Hello" in response.text
+
+
+def test_column_metadata(app_client):
+    response = app_client.get("/fixtures/roadside_attractions")
+    soup = Soup(response.body, "html.parser")
+    dl = soup.find("dl")
+    assert [(dt.text, dt.nextSibling.text) for dt in dl.findAll("dt")] == [
+        ("name", "The name of the attraction"),
+        ("address", "The street address for the attraction"),
+    ]
+    assert (
+        soup.select("th[data-column=name]")[0]["data-column-description"]
+        == "The name of the attraction"
+    )
+    assert (
+        soup.select("th[data-column=address]")[0]["data-column-description"]
+        == "The street address for the attraction"
+    )