PHP5 version of ADOdb_Active_Record now supports one-to-many relationships.

name属性に“id”を設定した場合，自動的にserialカラムとして設定されます。

とあるので, JOIN を使用できるのは PHP5 のみ?!

あと, id っていう名前の PRIMARY KEY を作るというのがデフォルトの規約みたいです.

14. Primary Keys ADOdb_Active_Record does not require the table to have a primary key. You can insert records for such a table, but you will not be able to update nor delete. Sometimes you are retrieving data from a view or table that has no primary key, but has a unique index. You can dynamically set the primary key of a table through the constructor, or using ADOdb_Active_Record::SetPrimaryKeys():

15. Retrieval of Auto-incrementing ID When creating a new record, the retrieval of the last auto-incrementing ID is not reliable for databases that do not support the Insert_ID() function call (check $connection->hasInsertID). In this case we perform a SELECT MAX($primarykey) FROM $table, which will not work reliably in a multi-user environment. You can override the ADOdb_Active_Record::LastInsertID() function in this case.

22. TableKeyHasMany For some classes, the mapping between class name and table name (which is the pluralised version) might not match or the primary key is not the default id. For example, the class name might be person, but the table name might be people. So we have 2 tables, people (parent table) and children (child table) linked by people.pid = children.person_id. Then you use the following static function ADODB_Active_Record::TableKeyHasMany($table, $tablePKey, $relation, $foreignKey = '', $foreignClass = 'ADODB_Active_Record') like this: ADODB_Active_Record::TableKeyHasMany('people', 'pid', 'children', 'person_id')

rhosoiさんは書きました：
使ったことはないのですが、rhacoというフレームワークに近いライブラリがあるそうで
http://rhaco-users.jp/
これにはO/Rマッパーも含まれているようです。
# rhaco自体は修正BSDライセンスだそうです

JOIN というか ClassBelongsTo() とか ClassHasMany() が PHP4 では使えないということみたいです。

これは

... snip

を読む限り id 以外のキーでも constructor でごにょごにょしてやれば使えます。