Fix #[changeset_for] with single field structs

This error occurred because we were always assuming the type and body to
work with tuples, and `Changeset` isn't implemented for `(A)`. We do the
same workaround in the generation for `insertable_into`. I'd rather just
add `(A)` to the tuple impls, but this is unfortunately impossible due
to rust-lang/rust#30762

CHANGELOG.md

@@ -24,6 +24,9 @@ for Rust libraries in [RFC #1105](https://github.com/rust-lang/rfcs/blob/master/
 * No longer generate invalid SQL when an optional update field is not the first
   field on a changeset. Fixes [#68](https://github.com/sgrif/diesel/issues/68).
 
+* `#[changeset_for]` can now be used with structs containing only a single field
+  other than `id`. Fixes [#66](https://github.com/sgrif/diesel/issues/66).
+
 ## [0.3.0] 2015-12-04
 
 ### Changed

diesel_codegen/src/update.rs

@@ -61,17 +61,26 @@ fn changeset_impl(
     let pk = model.primary_key_name();
     let attrs_for_changeset = model.attrs.iter().filter(|a| a.column_name != pk)
         .collect::<Vec<_>>();
-    let changeset_ty = builder.ty().tuple()
-        .with_tys(attrs_for_changeset.iter().map(|a| changeset_ty(cx, builder, table, a)))
-        .build();
-    let changeset_body = builder.expr().tuple()
-        .with_exprs(attrs_for_changeset.iter().map(|a| changeset_expr(cx, builder, table, a)))
-        .build();
+    let changeset_t;
+    let changeset_body;
+    if attrs_for_changeset.len() == 1 {
+        changeset_t = changeset_ty(cx, builder, table, attrs_for_changeset[0]);
+        changeset_body = changeset_expr(cx, builder, table, attrs_for_changeset[0]);
+    } else {
+        changeset_t = builder.ty().tuple()
+            .with_tys(attrs_for_changeset.iter()
+                      .map(|a| changeset_ty(cx, builder, table, a)))
+            .build();
+        changeset_body = builder.expr().tuple()
+            .with_exprs(attrs_for_changeset.iter()
+                        .map(|a| changeset_expr(cx, builder, table, a)))
+            .build();
+    }
     quote_item!(cx,
         impl<'a: 'update, 'update> ::diesel::query_builder::AsChangeset for
             &'update $struct_name
         {
-            type Changeset = $changeset_ty;
+            type Changeset = $changeset_t;
 
             fn as_changeset(self) -> Self::Changeset {
                 $changeset_body

diesel_tests/tests/update.rs

@@ -184,3 +184,25 @@ fn sql_syntax_is_correct_when_option_field_comes_mixed_with_non_option() {
     let expected_post = Post::new(post.id, sean.id, "Hello".into(), Some("earth".into()));
     assert_eq!(expected_post, post);
 }
+
+#[test]
+fn can_update_with_struct_containing_single_field() {
+    #[changeset_for(posts)]
+    struct SetBody {
+        body: String,
+    }
+
+    let connection = connection_with_sean_and_tess_in_users_table();
+    let sean = find_user_by_name("Sean", &connection);
+    let new_post = sean.new_post("Hello", Some("world"));
+    insert(&new_post).into(posts::table).execute(&connection).unwrap();
+
+    let changes = SetBody { body: "earth".into() };
+    let post = update(posts::table)
+        .set(&changes)
+        .get_result::<Post>(&connection)
+        .unwrap();
+
+    let expected_post = Post::new(post.id, sean.id, "Hello".into(), Some("earth".into()));
+    assert_eq!(expected_post, post);
+}