title: Doctrine Cascade Options for OneToMany
body: <p>I'm having a hard time making sense of the Doctrine manual's <a href="http://www.doctrine-project.org/docs/orm/2.1/en/reference/working-with-associations.html#transitive-persistence-cascade-operations" rel="noreferrer">explanation</a> of cascade operations and need someone to help me understand the options in terms of a simple ManyToOne relationship.</p>
  
  <p>In my application, I have a table/entity named Article that has a foreign key field referencing the 'id' field in a table/entity named Topic.</p>
  
  <p>When I create a new Article, I select the Topic from a dropdown menu. This inserts an integer into the 'topic_id' foreign key field in the Article table.</p>
  
  <p>I have the $topic association set up in the Article entity like this:</p>
  
  <pre><code>/**
   * @ManyToOne(targetEntity="Topic")
   * @JoinColumn(name="topic_id", referencedColumnName="id", nullable=false)
   */
  private $topic;
  </code></pre>
  
  <p>The Topic entity doesn't have any reciprocating annotation regarding the Article entity. Topics don't care what Articles reference them and nothing needs to happen to a Topic when an Article that references the Topic is deleted.</p>
  
  <p>Because I'm not specifying the cascade operation in the Article entity, Doctrine throws an error when I try to create a new Article: "A new entity was found through a relationship that was not configured to cascade persist operations. Explicitly persist the new entity or configure cascading persist operations on the relationship."</p>
  
  <p>So I know I need to choose a cascade operation to include in the Article entity, but how do I know which operation to choose in this situation?</p>
  
  <p>From reading the Doctrine manual, "detach" sounds like the right option. But researching others' similar questions <a href="https://stackoverflow.com/questions/7467982/error-saving-onetomany-and-manytomany-relationship-with-doctrine-2">here</a> and <a href="https://stackoverflow.com/questions/7467982/error-saving-onetomany-and-manytomany-relationship-with-doctrine-2">here</a> makes me think I want to use "persist" instead.</p>
  
  <p>Can anyone help me understand what "persist," "remove," "merge," and "detach" mean in terms of a simple ManyToOne relationship like the one I've described?</p>