Test Case:
1. Create JPA project with Hibernate Platform
2. Create 2 Entities with @ManyToMany mapping
3. Open Jpa Details view:
Failed: It should not be empty
4. create additional fieldin Entity without annotations
5. In Jpa Details view click on "here" link.
6. select ManyToMapping
7. Observe exception:

Caused by: java.lang.IllegalArgumentException: invalid mapping key: manyToMany
at org.eclipse.jpt.jpa.core.internal.context.java.AbstractJavaPersistentAttribute.getSpecifiedMappingDefinition(AbstractJavaPersistentAttribute.java:325)
at org.eclipse.jpt.jpa.core.internal.context.java.AbstractJavaPersistentAttribute.setMappingKey_(AbstractJavaPersistentAttribute.java:216)
at org.eclipse.jpt.jpa.core.internal.context.java.AbstractJavaPersistentAttribute.setMappingKey(AbstractJavaPersistentAttribute.java:195)
at org.eclipse.jpt.jpa.core.internal.context.java.AbstractJavaPersistentAttribute.setMappingKey(AbstractJavaPersistentAttribute.java:1)
at org.eclipse.jpt.jpa.ui.internal.details.PersistentAttributeMapAsComposite$1.morphMapping(PersistentAttributeMapAsComposite.java:82)