Let's say that we have two domain classes Book.groovy and Library.groovy.

// Book.groovy
package com.example.book
 
class Book {
    static constraint = {
        name blank: false, size: 2..255, unique: true
    }
    static belongsTo = [lib: Library]
    String name
}

// Library.groovy
package com.example.library
 
class Library {
    static hasMany = [book: Book, branch: Branch]
    static constraints = {
        name blank: false
        place blank: false
    }
    String name
    String place
}

To unit test Book class we need to create an object of Library class since Book 'belongsTo' library with a back reference.

So our unit test will be:

// BookUnitTests.groovy
package com.example.book
 
import grails.test.*
import com.example.library.*
 
class BookUnitTests extends GrailsUnitTestCase {
     
    def b    // book
    def lib  // library
 
    protected void setUp() {
        super.setUp()
        lib = new Library(name: "My Library", place: "XYZ Ave")
        mockForConstraintsTests(Book)
    }
 
    protected void tearDown() {
        super.tearDown()
    }
 
    void testPass() {
        b = new Book(name: "MyBook", lib: lib)  // pass the Library object to which the book belongs to.
        assert b.validate()
    }
}