Including SQL in Specifications

SqlSpecification.php

<?php
// This is another response to Mattias Verraes talk on unbreakable domain
// models (slides at http://verraes.net/2013/06/unbreakable-domain-models/)
// Near the end there's a bit where he suggests you implement business logic
// rules initially as code (i.e. an array filter) and secondly as sql.
// You can then test that these are consistent (though the tests in the talk
// don't show correctness, there needs to be a test against a known good result
// too).  But with that, it seems like a nice way to have testable SQL.

// A problem for me with the example (which is obviously shortened for the slide example)
// is that it shows hard-coded SQL strings.  SQL ought to be in one place.  But also,
// I want these things to be chainable (see https://gist.github.com/mattparker/9177040).
// So whatever's returned from asSql wants to be not a string of SQL.

// There's some ideas here, but not quite sure about them...

final class CustomerWith3OrdersIsPremium implements CustomerSpecification {

    /**
     * @return Select
     */
    public function asSql (Select $select) {

        // I still don't think this is quite right.
        // I don't really like that SQL string here.
        $select->join(new CustomerTable)
            ->where('summary_order_count >= 3');
        return $select;
    }

}

// or perhaps

final class CustomerWith3OrdersIsPremium implements CustomerSpecification {

    /**
     * @return Select
     */
    public function asSql (Select $select) {

        // This still isn't quite right.
        // It's better in that the SQL being written is now in
        // some kind of Select object that belongs to the table.
        // But instantiating the table here doesn't feel quite right.
        $customers = new CustomerTable;
        $customers->select()->orderCount(Select::GREATER_OR_EQUAL, 3);
        $select->where($customers);
        return $select;

    }

}

// or perhaps

final class CustomerWith3OrdersIsPremium implements CustomerSpecification {

    /**
     * @return QueryBuilder
     */
    public function asSql (QueryBuilder $builder) {

        // The Select object returned from this first line
        // knows about how to write SQL queries, join tables, etc.
        $customerSelect = $builder->getSelect(QueryBuilder::CUSTOMER);
        $customerSelect->hasOrderCount(Select::GREATER_OR_EQUAL, 3);

        // The QueryBuilder provides access to these Select objects,
        // and aggregates them and sends them off for execution
        $builder->where($customerSelect);
        return $builder;

    }

}