fix for GRAILS-6349 "Many-to-Many relationships with composite ids fail"

src/java/org/codehaus/groovy/grails/orm/hibernate/cfg/GrailsDomainBinder.java

@@ -742,7 +742,8 @@ private static void bindDependentKeyValue(GrailsDomainClassProperty property, De
 
         GrailsDomainClass refDomainClass = property.getDomainClass();
         final Mapping mapping = getMapping(refDomainClass.getClazz());
-        if (shouldCollectionBindWithJoinColumn(property) && hasCompositeIdentifier(mapping)) {
+        if ((shouldCollectionBindWithJoinColumn(property) && hasCompositeIdentifier(mapping)) ||
+        	(hasCompositeIdentifier(mapping) && property.isManyToMany())) {
             CompositeIdentity ci = (CompositeIdentity) mapping.getIdentity();
             bindCompositeIdentifierToManyToOne(property, key, ci, refDomainClass, EMPTY_PATH);
         }

src/test/org/codehaus/groovy/grails/orm/hibernate/ManyToManyCompositeIdTests.groovy

@@ -0,0 +1,107 @@
+package org.codehaus.groovy.grails.orm.hibernate;
+
+import junit.framework.TestCase;
+
+class ManyToManyCompositeIdTests extends AbstractGrailsHibernateTests {
+
+	void testManyToManyDomain() {
+		def authorDomain = ga.getDomainClass("ManyToManyCompositeIdAuthor")
+		def bookDomain = ga.getDomainClass("ManyToManyCompositeIdBook")
+
+		def books = authorDomain?.getPropertyByName("books")
+		def authors = bookDomain?.getPropertyByName("authors")
+
+		assert books?.isManyToMany()
+		assert authors?.isManyToMany()
+		assert !books?.isOneToMany()
+		assert !authors?.isOneToMany()
+	}
+	void testManyToManyMapping() {
+		def Author = ga.getDomainClass("ManyToManyCompositeIdAuthor").clazz
+		def Book = ga.getDomainClass("ManyToManyCompositeIdBook").clazz
+
+        def a = Author.newInstance(name:"Stephen King", family:10, child:1)
+
+		a.addToBooks(Book.newInstance(title:"The Shining", isbn:5001, edition:2))
+         .addToBooks(Book.newInstance(title:"The Stand", isbn:3402, edition:4))
+         .save(true)
+		assertEquals 2, Book.list().size()
+
+		def b = Book.get(Book.newInstance(isbn:5001, edition:2))
+		assert b
+        assertNotNull b.authors
+        assertEquals 1, b.authors.size()
+
+		a = Author.get(Author.newInstance(family:10, child:1))
+		assert a
+        assertNotNull a.books
+        assertEquals 2, a.books.size()
+
+
+		assertEquals b, a.books.find { (it.isbn == 5001) && (it.edition == 2) }
+		this.session.flush()
+		session.clear()
+
+		a = Author.get(Author.newInstance(family:10, child:1))
+		assert a
+		assert a.books
+
+		b = Book.get(Book.newInstance(isbn:5001, edition:2))
+		assert b
+		assert b.authors
+
+        // now let's test the database state
+        def c = session.connection()
+
+        def ps = c.prepareStatement("select * from many_to_many_composite_id_author_books order by many_to_many_composite_id_book_isbn")
+        def rs = ps.executeQuery()
+        assert rs.next()
+
+        assertEquals 10,rs.getInt("many_to_many_composite_id_author_family")
+        assertEquals 1,rs.getInt("many_to_many_composite_id_author_child")
+        assertEquals 3402,rs.getInt("many_to_many_composite_id_book_isbn")
+        assertEquals 4,rs.getInt("many_to_many_composite_id_book_edition")
+
+        assert rs.next()
+        assertEquals 10,rs.getInt("many_to_many_composite_id_author_family")
+        assertEquals 1,rs.getInt("many_to_many_composite_id_author_child")
+        assertEquals 5001,rs.getInt("many_to_many_composite_id_book_isbn")
+        assertEquals 2,rs.getInt("many_to_many_composite_id_book_edition")
+    }
+
+    void onSetUp() {
+		this.gcl.parseClass('''
+import grails.persistence.*
+
+@Entity
+class ManyToManyCompositeIdBook implements Serializable {
+	Long isbn
+	Long edition
+	String title
+	static belongsTo = ManyToManyCompositeIdAuthor
+	static hasMany = [authors:ManyToManyCompositeIdAuthor]
+
+	static mapping = {
+		id composite:['isbn', 'edition'], generator:'assigned'
+	}
+}
+
+@Entity
+class ManyToManyCompositeIdAuthor implements Serializable {
+	Long family
+	Long child
+    String name
+	static hasMany = [books:ManyToManyCompositeIdBook]
+
+	static mapping = {
+		id composite:['family', 'child'], generator:'assigned'
+	}
+}
+'''
+		)
+	}
+
+	void onTearDown() {
+
+	}
+}