Best practice in defining <entity /> and <column /> values is to adhere to Java naming standards. This typically means using mixed case, capital-first for the entity but lowercase first for the column. This defines an entity according to Java POJO practices which is what your entity will appear as to regular Java code.

Your DBA may have different requirements for tables and column names. For those cases, you should use the db-name attribute to specify a name format that adheres to the DBA requirements.

Could this mean a db-name attribute on each <column />? Sure can, and I've done it.

The underscore is appended to certain names because while your database may be okay with a column named "id", in others it is a reserved word and would cause Liferay issues if trying to use. Since Liferay is database agnostic, it treats all of these various reserved words in the same way, appending an underscore to the name. The db-name attribute is the only way to bypass that, but if you deploy to a DB where "id" is a reserved word, you will be stuck dealing with the error.

OK, I got that! Thanks.
But regarding this error:
org.hibernate.QueryException: could not resolve property: manufacturer_id of: [myPackage].model.impl.DeviceImpl
doing something like
            DynamicQuery deviceQuery = DeviceLocalServiceUtil.dynamicQuery();
            deviceQuery.add(RestrictionsFactoryUtil.eq("manufacturer_id", manufacturer_Id))
it turns out that it's not working because manufacturer_id is part of a composite primary key as defined in the service.xml like
        <!-- PK fields -->
        <column name="manufacturer_id" db-name="manufacturer_id" primary="true" type="String" />
        <column name="name" db-name="name" primary="true" type="String" />

This generates a module.hbm.xml that looks with this:
        <composite-id class="[myPackage].service.persistence.DevicePK" name="primaryKey">
            <key-property access="com.liferay.portal.dao.orm.hibernate.LiferayPropertyAccessor" name="manufacturer_id" type="com.liferay.portal.dao.orm.hibernate.StringType" />
            <key-property access="com.liferay.portal.dao.orm.hibernate.LiferayPropertyAccessor" name="name" type="com.liferay.portal.dao.orm.hibernate.StringType" />
        </composite-id>

And now I'm going crazy because the dynamicQuery is not working (grrrr)

Already tried withevicePK devicePK = new DevicePK();               
.setProjection(ProjectionFactoryUtil.property(devicePK.getManufacturerId())) .setProjection(ProjectionFactoryUtil.property(devicePK.getName()));
to simply retriev the values from the DB. But that causes a NullpointerException.

How to use composite primary keys in dynamic queries?

Liferay ships a usage for JAXB in the item. We simply need to design the JVM to search for it as opposed to attempting to search for the default old one. Simply setting the property javax.xml.bind.JAXBContextFactory=com.sun.xml.bind.v2.ContextFactory ought to do it. You ought to have the option to set it on your condition or executable for the JVM that begins Liferay. It ought to be set of course if later Liferay renditions.

Hello. Did you get the answer to your question? I am in the same situation