Custom error message for Postgresql CHECK IN list constraint

Question

I would like to create a more specific error message for Postgres CHECK IN violations. So for example a violation of the following CHECK constraint on a column:

management_zone varchar(15) NOT NULL CHECK (management_zone IN ('Marine', 'Terrestrial') ),

should return a custom error message such as ie.: "Hint: Check spelling. Only allowed inputs are: 'Marine', 'Terrestrial'.

The best solution I have seen so far solves it by using the error message as the name of the check constraint, ie

ADD CONSTRAINT c_managementzone_@Check_spelling._Only_allowed_inputs_are:_'Marine',_'Terrestrial' CHECK (management_zone IN ('Marine', 'Terrestrial') ),

and then applying a function that fixes the error message after the @ sign by replacing the underscores with space to make it more readable. The function is then called using a try and catch statement.

This code however is in t-sql which I am unfamiliar with, and I also don't know enough PL/pgSQL to be able to convert and replicate it. I therefore wonder if anyone can suggest how something similar can be done in postgres, ex. using a function and a trigger?

The t-sql code and explanation is available from here and I copy paste it below: https://social.technet.microsoft.com/wiki/contents/articles/29187.t-sql-error-handling-for-check-constraints.aspx

CREATE FUNCTION dbo.ufnGetClearErrorMessage2()
RETURNS NVARCHAR(4000)
AS
BEGIN
    DECLARE @Msg NVARCHAR(4000) = ERROR_MESSAGE() ;
    DECLARE @ErrNum INT = ERROR_NUMBER() ;
    DECLARE @ClearMessage NVARCHAR(4000) ;
    IF @ErrNum = 547
        BEGIN
            /*--how to find @ClearMessage:
            SELECT @msg ,
               CHARINDEX('@', @msg) ,
               RIGHT(@msg, LEN(@msg) - CHARINDEX('@', @msg)) ,
               CHARINDEX('"', RIGHT(@msg, LEN(@msg) - CHARINDEX('@', @msg))) ,
               LEFT(RIGHT(@msg, LEN(@msg) - CHARINDEX('@', @msg)), CHARINDEX('"', RIGHT(@msg, LEN(@msg) - CHARINDEX('@', @msg))) - 1 ) ,
               REPLACE(LEFT(RIGHT(@msg, LEN(@msg) - CHARINDEX('@', @msg)), CHARINDEX('"', RIGHT(@msg, LEN(@msg) - CHARINDEX('@', @msg))) - 1 ), '_', SPACE(1)) + '.'
        */
            SELECT @ClearMessage = @Msg + CHAR(13) +
            REPLACE(LEFT(RIGHT(@msg, LEN(@msg) - CHARINDEX('@', @msg)), CHARINDEX('"', RIGHT(@msg, LEN(@msg) - CHARINDEX('@', @msg))) - 1 ), '_', SPACE(1)) + '.'

        END
    ELSE
        SET @ClearMessage = @Msg ;

    RETURN @ClearMessage ;
END

Try and catch:

BEGIN TRY
INSERT  dbo.Book
        ( BookId, WritingDate, publishDate )
VALUES  ( 1, GETDATE(), GETDATE() + 1 )
END TRY
BEGIN CATCH

DECLARE @Msg NVARCHAR(4000) = dbo.ufnGetClearErrorMessage2();
THROW 60001, @Msg, 1;
END CATCH

Any advice much apreciated.

Answer 1

If you can live with a slightly different check constraint, you can do the following:

Create a function that checks the values:

create function check_zone(p_input text)
  returns boolean
as
$$
declare
  l_allowed text[] := array['Marine', 'Terrestrial'];
begin
  if p_input = any(l_allowed) then 
    return true;
  end if;
  raise 'The only allowed values are: %', array_to_string(l_allowed, ', ');
end;
$$
language plpgsql
immutable;

And then use that function instead of an IN condition:

create table data
(
  management_zone text not null,
  CONSTRAINT check_zone CHECK (check_zone(management_zone))
);

The following INSERT

insert into data values ('foo');

will result in:

ERROR: The only allowed values are: Marine, Terrestrial