Add test for removeFrom with the in memory impl

grails-datastore-gorm-test/src/test/groovy/grails/gorm/tests/RemoveFromSpec.groovy

@@ -0,0 +1,59 @@
+package grails.gorm.tests
+
+import grails.gorm.annotation.Entity
+import spock.lang.Issue
+
+class RemoveFromSpec extends GormDatastoreSpec {
+
+    @Issue("https://github.com/grails/grails-data-mapping/issues/998")
+    void "test removeFrom clears the back reference"() {
+        given:
+        new AuthorTest(name: "Joe").addToBooks(title: "Ready Player One").addToBooks(title: "Total Recall").save(flush: true, failOnError: true)
+        new BookTest(title: "Unrelated").save(flush: true, failOnError: true)
+        session.flush()
+        session.clear()
+        AuthorTest author = AuthorTest.first()
+        BookTest book = author.books.find { it.title == "Total Recall" }
+
+        expect:
+        author.books.size() == 2
+
+        when:
+        author.removeFromBooks(book)
+        author.save()
+        book.save()
+        session.flush()
+        session.clear()
+        author = AuthorTest.first()
+
+        then:
+        author.books.size() == 1
+        BookTest.count == 2
+    }
+
+    List getDomainClasses() {
+        [AuthorTest, BookTest]
+    }
+
+}
+
+@Entity
+class AuthorTest {
+    String name
+
+    static hasMany = [books: BookTest]
+
+    static constraints = {
+    }
+}
+
+@Entity
+class BookTest {
+    String title
+
+    static belongsTo = [author: AuthorTest]
+
+    static constraints = {
+        author nullable: true
+    }
+}