added to_json function

diesel/src/pg/expression/expression_methods.rs

@@ -2,7 +2,7 @@
 
 pub(in crate::pg) use self::private::{
     ArrayOrNullableArray, InetOrCidr, JsonIndex, JsonOrNullableJsonOrJsonbOrNullableJsonb,
-    JsonRemoveIndex, JsonbOrNullableJsonb, MultirangeOrNullableMultirange,
+    JsonRemoveIndex, JsonbOrNullableJsonb, MaybeNullableValue, MultirangeOrNullableMultirange,
     MultirangeOrRangeMaybeNullable, RangeHelper, RangeOrNullableRange, TextOrNullableText,
 };
 use super::date_and_time::{AtTimeZone, DateTimeLike};
@@ -3417,8 +3417,8 @@ where
 
 pub(in crate::pg) mod private {
     use crate::sql_types::{
-        Array, Binary, Cidr, Inet, Integer, Json, Jsonb, Multirange, Nullable, Range, SingleValue,
-        SqlType, Text,
+        Array, Binary, Cidr, Inet, Integer, Json, Jsonb, MaybeNullableType, Multirange, Nullable,
+        Range, SingleValue, SqlType, Text,
     };
     use crate::{Expression, IntoSql};
 
@@ -3699,4 +3699,17 @@ pub(in crate::pg) mod private {
 
     impl BinaryOrNullableBinary for Binary {}
     impl BinaryOrNullableBinary for Nullable<Binary> {}
+
+    pub trait MaybeNullableValue<T>: SingleValue {
+        type Out: SingleValue;
+    }
+
+    impl<T, O> MaybeNullableValue<O> for T
+    where
+        T: SingleValue,
+        T::IsNull: MaybeNullableType<O>,
+        <T::IsNull as MaybeNullableType<O>>::Out: SingleValue,
+    {
+        type Out = <T::IsNull as MaybeNullableType<O>>::Out;
+    }
 }

diesel/src/pg/expression/functions.rs

@@ -3,6 +3,7 @@
 use super::expression_methods::InetOrCidr;
 use crate::expression::functions::define_sql_function;
 use crate::pg::expression::expression_methods::ArrayOrNullableArray;
+use crate::pg::expression::expression_methods::MaybeNullableValue;
 use crate::pg::expression::expression_methods::MultirangeOrNullableMultirange;
 use crate::pg::expression::expression_methods::MultirangeOrRangeMaybeNullable;
 use crate::pg::expression::expression_methods::RangeOrNullableRange;
@@ -1423,3 +1424,49 @@ define_sql_function! {
     /// ```
     fn array_upper<Arr: ArrayOrNullableArray + SingleValue>(array: Arr, dimension: Integer) -> Nullable<Integer>;
 }
+
+#[cfg(feature = "postgres_backend")]
+define_sql_function! {
+    /// Converts any SQL value to json
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # include!("../../doctest_setup.rs");
+    /// #
+    /// # fn main() {
+    /// #     #[cfg(feature = "serde_json")]
+    /// #     run_test().unwrap();
+    /// # }
+    /// #
+    /// # #[cfg(feature = "serde_json")]
+    /// # fn run_test() -> QueryResult<()> {
+    /// #     use diesel::dsl::to_json;
+    /// #     use serde_json::{json, Value};
+    /// #     use diesel::sql_types::{Integer, Array, Json, Text, Nullable};
+    /// #     let connection = &mut establish_connection();
+    /// let result = diesel::select(to_json::<Integer, _>(1))
+    ///     .get_result::<Value>(connection)?;
+    ///
+    /// assert_eq!(json!(1), result);
+    ///
+    /// let result = diesel::select(to_json::<Array<Text>, _>(vec!["abc", "xyz"]))
+    ///     .get_result::<Value>(connection)?;
+    ///
+    /// assert_eq!(json!(["abc", "xyz"]), result);
+    ///
+    /// let result = diesel::select(to_json::<Array<Nullable<Text>>, _>(Vec::<String>::new()))
+    ///     .get_result::<Value>(connection)?;
+    ///
+    /// assert_eq!(json!([]), result);
+    ///
+    /// let result = diesel::select(to_json::<Nullable<Text>, _>(None::<String>))
+    ///     .get_result::<Option<Value>>(connection)?;
+    ///
+    /// assert!(result.is_none());
+    ///
+    /// #     Ok(())
+    /// # }
+    /// ```
+    fn to_json<E: MaybeNullableValue<Json>>(e: E) -> E::Out;
+}

diesel/src/pg/expression/helper_types.rs

@@ -461,3 +461,8 @@ pub type array_positions<A, E> =
 #[allow(non_camel_case_types)]
 #[cfg(feature = "postgres_backend")]
 pub type array_ndims<A> = super::functions::array_ndims<SqlTypeOf<A>, A>;
+
+/// Return type of [`to_json(element)`](super::functions::to_json())
+#[allow(non_camel_case_types)]
+#[cfg(feature = "postgres_backend")]
+pub type to_json<E> = super::functions::to_json<SqlTypeOf<E>, E>;

diesel_derives/tests/auto_type.rs

@@ -432,6 +432,7 @@ fn postgres_functions() -> _ {
         array_position_with_subscript(pg_extras::array, pg_extras::id, pg_extras::id),
         array_positions(pg_extras::array, pg_extras::id),
         array_ndims(pg_extras::array),
+        to_json(pg_extras::id),
     )
 }