[TYPO3-core] RFC: Bidirectioanl MM relations

[Ingmar Schlecht]

This is a SVN patch request.

Problem:
Currently you can edit mm-relations in the Backend from one side of the
relation only.

Example:
Say you have two tables, "persons" and "companies".
Now, if you'd want to express the employment relation between those, you
would either create a field called "employers" in the persons table or
you would create an "employees" field in the companies table.
Until now you couldn't make that relation editable from both sides, even
though it would technically be possible if an mm-table is used.

Solution:
We (Sebastian and me) made it possible to configure the same relation
from both sides exactly the same way. There is only one difference
between the configuration on the "foreign" side of the relation compared
to the configuration on the "local" side (You still need to decide which
side you want to call "local", even though there is no longer a real
difference between the "local" and "foreign" side): The foreign-side
needs to have the following defined in TCA:
    "MM_opposite_field" => 'employees',

Here's an example configuration:

    $TCA["companies"] = Array (
        [...]
        "columns" => Array (
            [...]
            "employees" => Array (
                "exclude" => 1,
                "label" => "Employees",
                "config" => Array (
                    "type" => "select",
                    "foreign_table" => "persons",
                    "size" => 5,
                    "MM" => "companies_employees_mm",
                )
            )
        )
    );

    $TCA["persons"] = Array (
        [...]
        "columns" => Array (
            [...]
            "employers" => Array (
                "exclude" => 1,
                "label" => "Employers",
                "config" => Array (
                    "type" => "select",
                    "foreign_table" => "companies",
                    "size" => 5,
                    "MM" => "companies_employees_mm",

                        // New:
                    "MM_opposite_field" => 'employees',
                )
            )
        )
    );

And the tables.sql file:
    #
    # Table structure for table 'tx_mmftest_company_employees_mm'
    #
    #
    CREATE TABLE companies_employees_mm (
        uid_local int(11) DEFAULT '0' NOT NULL,
        uid_foreign int(11) DEFAULT '0' NOT NULL,
        tablenames varchar(30) DEFAULT '' NOT NULL,
        sorting int(11) DEFAULT '0' NOT NULL,

            # New:
        sorting_foreign int(11) DEFAULT '0' NOT NULL,

        KEY uid_local (uid_local),
        KEY uid_foreign (uid_foreign)
    );

Difference between René's patch from 19.01.2006 and this one:
René's patch didn't only make it possible to edit mm relations from both
sides of the relation, but also added the possibility to store *multiple
different* relation-types between the same set of tables into *one*
mm-table. For example with René's Patch it was possible to store the
employment relation as well as the customership relation (both are
companies<=>persons relations) into the same mm-table by what he called
"ident strings".
However, we considered this feature to be not so important because you
could just as well create a different mm-table for each relation-type. A
positive side-effect of dropping that feature was that our patch only
touches the "foreign" side and doesn't change any behavior on the
"normal" side of the relation, so existing normal mm-fields are not
changed in any way, which hopefully reduces the amount of bugs...

Now, for those who want to review the patch:
What we mainly needed to do was modifying t3lib_loaddbgroup to use the
uid_local field to store the foreign uid and the uid_foreign field to
store the local uid in case we are lookign at a relation from the
"foreign side". Unfortunately t3lib_loaddbgroup didn't know the TCA
configuration of the field it was working on until now, but we needed
it. Therefore we introduced a new parameter $conf to the start()
function of t3lib_loaddbgroup containing the TCA field configuration.
Then we needed to modify all places where t3lib_loaddbgroup::start() was
called, to make sure the $conf parameter was passed.

Another problem we faced was sorting: The sorting on the one side of the
relation (employees of companies) doesn't have anything to do with the
sorting on the other side of the relation (employers of persons).
Therefore we introduced another field sorting_foreign to handle sorting
on the foreign side. If a new relation is added on the one side (the
person selects a new employer), that new entity (the person) is listed
on top on the other side (the company side). Whether new items added on
the other side are listed on top or at the end of the list can be
ajusted with the MySQL default value of the sorting columns.

I would like the patch to enter trunk rather sooner then later to have a
long testing period until 4.1 is released. You can test the new feature
with the attached test extension.

cheers
Ingmar
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