many-to-many obviously stores relations in wrong columns

steps to reproduce the error:

1. install testapp attached with this report grails version is 0.4.1 but also occures on 0.4
2. configure database (i used mysql at localhost databasename tester user root no password)
3. run application
4. create two categories (important to see the error)
5. create 1 product
6. edit product and add two categories
7. check table product_category should look like
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