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use backend::Backend;
use query_builder::AstPass;
use query_source::QuerySource;
use result::QueryResult;

/// Types which can be passed to
/// [`update.set`](/diesel/query_builder/struct.IncompleteUpdateStatement.html#method.set).
///
/// ### Deriving
///
/// This trait can be automatically derived using `diesel_codegen` by adding
/// `#[derive(AsChangeset)]` to your struct.  Structs which derive this trait
/// must be annotated with `#[table_name = "something"]`. If the field name of
/// your struct differs from the name of the column, you can annotate the field
/// with `#[column_name = "some_column_name"]`.
///
/// 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")]`.
pub trait AsChangeset {
    type Target: QuerySource;
    type Changeset;

    fn as_changeset(self) -> Self::Changeset;
}

/// Apps should not need to concern themselves with this trait.
pub trait Changeset<DB: Backend> {
    fn is_noop(&self) -> bool;
    fn walk_ast(&self, pass: AstPass<DB>) -> QueryResult<()>;
}

impl<T: AsChangeset> AsChangeset for Option<T> {
    type Target = T::Target;
    type Changeset = Option<T::Changeset>;

    fn as_changeset(self) -> Self::Changeset {
        self.map(|v| v.as_changeset())
    }
}

impl<T: Changeset<DB>, DB: Backend> Changeset<DB> for Option<T> {
    fn is_noop(&self) -> bool {
        self.is_none()
    }

    fn walk_ast(&self, out: AstPass<DB>) -> QueryResult<()> {
        match *self {
            Some(ref c) => c.walk_ast(out),
            None => Ok(()),
        }
    }
}