Documentation

General Constraints

Sometimes you need to put a general constraint onto a column's value that requires conditional processing and specialized error messages. This can be done with a chain, using almost the same syntax as calculated values.

Consider the example of a shopping cart that offers some customers a steep discount. Others may qualify for free shipping. However, no customer can get both. Here is how to code this in Andromeda:

table orderlines:
    module: ordering
    description: Cart Items
    
    column discount:
        auto: fetch,customers.discount
    column flag_freeship:
        suffix: _freeship
        chain cons:
            test 00:
                compare: @flag_freeship = N
                return:
            test 01:
                compare: @discount <= 20
                return:
            test 02:
                return: Discount exceeds 20%, free shipping not allowed

Now let us review the basic syntax of the 'chain'

• A chain is a property of a column in a table.
• To make a constraint, begin with 'chain cons', this is to distinguish a constraint chain from a calculation chain.
• The constraint functions by evaluating a sequence of tests. You may number the tests however you like, but by convention we use 01, 02, etc.
• Each test begins with a 'compare' operation. If the compare returns true, the chain ends, returning the 'return' value.
• Execution continues until the chain reaches a true condition or runs out of tests. If it runs out of tests the constraint passes - the value is allowed.
• An empty return value means the constraint passed - the value is allowed.
• A non-empty value means the constraint failed, and the text is taken to be the error message.

Only one constraint chain is allowed per column. However, constraint chains have a peculiar property, they do not actually need to be defined on the column whose value you are testing -- they are really table-level definitions that we define for convenience on columns. Therefore, if required, you can put other constraints on other columns.

Compare and Return Syntax is Not Nice

The syntax for the compare expressions is not very flexible, by the standards of most languages and by the rest of Andromeda. We hope to rectify this in a future release, but at least through Release 1 the following restrictions apply:

• There is only one comparison allowed per expression. Expressions like '@a = @b OR @c = @d' are not allowed.
• Column names are preceeded by an @ sign, everything else is considered a string or numeric literal.
• Only one space is allowed between each argument. Andromeda does not trap for this and does not correct mistakes in this rule.

More Information on Comparison Operators

A list of supported comparison operators is available on the page on Calculated Values.