title: Remove all related children in Symfony entity ManyToOne
body: <p>That is my relations:</p>
  
  <p>in CategoryField Entity: </p>
  
  <pre><code>manyToOne:
      category:
          targetEntity: Category
          inversedBy: fields
          joinColumn:
              name: category_id
              referencedColumnName: id
  </code></pre>
  
  <p>in Category:</p>
  
  <pre><code>oneToMany:
      fields:
          orderBy: { 'name': 'ASC' }
          targetEntity: CategoryField
          mappedBy: category
          cascade: ['remove']
  </code></pre>
  
  <p>I'm trying to remove all fields when I save category(before inserts new fields):</p>
  
  <pre><code>if (!$category = $categoryRepo-&gt;find($item_id)) {
      $category = new Category();
  } else {
      $category-&gt;clearFields();
  }
  </code></pre>
  
  <p>That is clear fields method in Category entity:</p>
  
  <pre><code>public function clearFields() {
      $this-&gt;fields-&gt;clear();
  }
  </code></pre>
  
  <p>Fields does not remove and new fields inserts in related table. That's makes duplicates.</p>
  
  <p>How can I remove all related records(fields) with my association?</p>