Hey, I'm working on a problem where users need to be able to dynamically add new fields/properties (columns) to persistent entities. I've been researching this for awhile now, and seem there were various solutions. The one I'm using now (prototyping) involves directly using the Hibernate API, i.e.



I was thinking this would be a way for me to add the columns and properties at runtime (or when the session factory is created.) A few things I have yet to find answers on:
1. How do you set a property on a class you generate at runtime with Hibernate? I'm guessing it involves reflection, but not sure how.
2. Is there a file that defines the SqlType's that are valid to use with Hibernate?
3. Should I be setting the column on the Table? Or should I be creating a Property instead?

The other issue with this problem (I believe) will be updating the database schema when the user adds a new field. I think I can make this work with LiquiBase, but haven't implemented anything yet.

If anyone has any ideas or suggestions on this, I would be grateful. Thanks everyone

If I were you, I would throw out Hibernate altogether for such use case. Do not think it is capable for anything like that.

We are doing something similar in our project. There are a lot of things to hook up to get everything working. I'll try to outline what we are doing and post some links to our code. Hope it will help you.

Basically we have an XML configuration that holds admin-defined properties. It is currently limited to a few simple data types (eg. string, int, float, boolean, etc.). The default configuration file looks like this: http://base.thep.lu.se/browser/branches ... erties.xml and it's use is documented here: http://base.thep.lu.se/chrome/site/late ... rties.html

When starting up we read in the XML file and store as ExtendedProperty objects. Those are then used to create Hibernate Property mappings that are programmatically added to a PersistentClass:

http://base.thep.lu.se/browser/branches ... .java#L272 and
http://base.thep.lu.se/browser/branches ... .java#L351

The API provided by Hibernate in this area is not a public API and it has happened that things break between Hibernate versions. The Javadoc is not very complete either so it is a lot of trial-and-error to get things working. Another approach (better?) would be to generate an XML-string identical to a regular hbm.xml file and then add that string to the configuration with Configuration.addDocument(). We are using this approach for some other entities (http://base.thep.lu.se/browser/branches ... .java#L298). I'm not sure why we don't use this approach in the first case as well...

To get/set values for the configurable properties we use a custom PropertyAccessor implementation (http://base.thep.lu.se/browser/branches ... essor.java) that re-direct set/get calls to a generic pair of methods that takes the property name as an argument. Inside the implementation we use a Map to store the values: (http://base.thep.lu.se/browser/branches ... .java#L195

This should complete the outline. I guess there are a lot of details not covered.

Thank you both for your responses.

nordborg: Very interesting solution and thanks for the code samples. I am currently looking over the code you linked and will continue to look at how you accomplished this. We are currently using annotations, but it seems that possibly manipulating a mapping file may be easier in the long run to maintain. The approach would be to read in the original mapping file, then add any fields that were defined dynamically, then call the addDocument method?

Was wondering though, how are you updating the database schema when a new column is added to a table? In other words, are you physically altering the table schema to add a new column?

I imagine I'll encountered a few more problems along the way, so hopefully I can draw on your experience :). Thanks again

We are using the schema update tool built into Hibernate. It's more or less:



This usually works well for adding columns. It doesn't support removing a column, so that needs to be done manually. As far as I know we have not had any real use case that needed to remove a column.