added to_json function

diesel/src/pg/expression/functions.rs

@@ -1423,3 +1423,56 @@ 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() {
+    /// #     run_test().unwrap();
+    /// # }
+    /// #
+    /// # fn run_test() -> QueryResult<()> {
+    /// #     use diesel::dsl::to_json;
+    /// #     use serde_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)?;
+    ///
+    /// let expected: Value = serde_json::from_str("1").unwrap();
+    /// assert_eq!(expected, result);
+    ///
+    /// let result = diesel::select(to_json::<Array<Integer>, _>(vec![1, 2, 3]))
+    ///     .get_result::<Value>(connection)?;
+    ///
+    /// let expected: Value = serde_json::from_str("[1, 2, 3]").unwrap();
+    /// assert_eq!(expected, result);
+    ///
+    /// let result = diesel::select(to_json::<Array<Text>, _>(vec!["abcd", "xyz"]))
+    ///     .get_result::<Value>(connection)?;
+    ///
+    /// let expected: Value = serde_json::from_str(r#"["abcd", "xyz"]"#).unwrap();
+    /// assert_eq!(expected, result);
+    ///
+    /// let result = diesel::select(to_json::<Array<Nullable<Text>>, _>(Vec::<String>::new()))
+    ///     .get_result::<Value>(connection)?;
+    ///
+    /// let expected: Value = serde_json::from_str("[]").unwrap();
+    /// assert_eq!(expected, result);
+    ///
+    /// let result = diesel::select(to_json::<Nullable<Array<Text>>, _>(None::<Vec<String>>))
+    ///     .get_result::<Value>(connection);
+    ///
+    /// assert!(result.is_err());
+    ///
+    /// #     Ok(())
+    /// # }
+    /// ```
+    fn to_json<E: SingleValue>(e: E) -> Json;
+}

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),
     )
 }