Tidy up units support

* Add units to exported JSON
* Units key in metadata skeleton
* Docs

datasette/app.py

@@ -84,6 +84,12 @@ class BaseView(RenderMixin):
         self.page_size = datasette.page_size
         self.max_returned_rows = datasette.max_returned_rows
 
+    def table_metadata(self, database, table):
+        "Fetch table-specific metadata."
+        return self.ds.metadata.get(
+            'databases', {}
+        ).get(database, {}).get('tables', {}).get(table, {})
+
     def options(self, request, *args, **kwargs):
         r = response.text('ok')
         if self.ds.cors:
@@ -449,11 +455,6 @@ class DatabaseDownload(BaseView):
 
 
 class RowTableShared(BaseView):
-    def table_metadata(self, database, table):
-        return self.ds.metadata.get(
-            'databases', {}
-        ).get(database, {}).get('tables', {}).get(table, {})
-
     def sortable_columns_for_table(self, name, table, use_rowid):
         table_metadata = self.table_metadata(name, table)
         if 'sortable_columns' in table_metadata:
@@ -890,6 +891,7 @@ class TableView(RowTableShared):
             'filtered_table_rows_count': filtered_table_rows_count,
             'columns': columns,
             'primary_keys': pks,
+            'units': self.table_metadata(name, table).get('units', {}),
             'query': {
                 'sql': sql,
                 'params': params,
@@ -959,6 +961,7 @@ class RowView(RowTableShared):
             'columns': columns,
             'primary_keys': pks,
             'primary_key_values': pk_values,
+            'units': self.table_metadata(name, table).get('units', {})
         }
 
         if 'foreign_key_tables' in (request.raw_args.get('_extras') or '').split(','):

datasette/cli.py

@@ -160,6 +160,7 @@ def skeleton(files, metadata, sqlite_extensions):
                     'license_url': None,
                     'source': None,
                     'source_url': None,
+                    'units': {}
                 } for table_name in (info.get('tables') or {})
             }
         }

docs/metadata.rst

@@ -55,6 +55,35 @@ You can also provide metadata at the per-database or per-table level, like this:
 
 Each of the top-level metadata fields can be used at the database and table level.
 
+Specifying units for a column
+-----------------------------
+
+Datasette supports attaching units to a column, which will be used when displaying
+values from that column. SI prefixes will be used where appropriate.
+
+Column units are configured in the metadata like so::
+
+    {
+        "databases": {
+            "database1": {
+                "tables": {
+                    "example_table": {
+                        "units": {
+                            "column1": "metres",
+                            "column2": "Hz"
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+Units are interpreted using Pint_, and you can see the full list of available units in
+Pint's `unit registry`_.
+
+.. _Pint: https://pint.readthedocs.io/
+.. _unit registry: https://github.com/hgrecco/pint/blob/master/pint/default_en.txt
+
 Setting which columns can be used for sorting
 ---------------------------------------------
 
@@ -119,7 +148,8 @@ This will create a ``metadata.json`` file looking something like this::
                         "license": null,
                         "license_url": null,
                         "source": null,
-                        "source_url": null
+                        "source_url": null,
+                        "units": {}
                     }
                 }
             },