update and set null: There are no changes to save. This query cannot be built

[fluxxu]

Which versions of Rust and Diesel are you using?

rustc 1.19.0-nightly (06fb4d256 2017-04-30)
diesel 0.12

Which feature flags are you using?

postgres, chrono, serde

What are you trying to accomplish?

generate a query to update fields to null
e.g.

UPDATE "tablename" SET "value" = NULL WHERE "id" = ?

What is the full error you are seeing?

panicked at 'called `Result::unwrap()` on an `Err` value: Error(DbResult(QueryBuilderError(StringError("There are no changes to save. This query cannot be built"))), State { next_error: None, backtrace: None
 })', /checkout/src/libcore/result.rs:859

Because None is a noop:

diesel/diesel/src/query_builder/update_statement/changeset.rs

Line 45 in e52d171

fn is_noop(&self) -> bool {

And struct with only None values can not pass this:

diesel/diesel/src/query_builder/update_statement/mod.rs

Line 58 in d44c50f

if self.values.is_noop() {

How can we reproduce this?

define a struct with only Option fields and derive AsChangeset

  #[derive(AsChangeset)]
  #[table_name="tablename"]
  struct Update {
    value: Option<i32>,
  }

Then update using this struct

  diesel::update(tablename::table.filter(dsl::id.eq(id)))
    .set(&Update {
        value: None
    })
    .execute(conn)?;

Please quote as much of your code as needed to reproduce (best link to a public repository or a Gist)

N/A

Please post as much of your database schema as is relevant to your error

N/A

[fluxxu]

http://docs.diesel.rs/diesel/query_builder/trait.AsChangeset.html

By default, any Option fields on the struct are skipped if their value is None. If you would like to assign NULL to the field instead, you can annotate your struct with #[changeset_options(treat_none_as_null = "true")].