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copied!<p>Short answer. Change <code>RecipeJDO.setIngredients()</code> to this:</p> <pre><code>public void setIngredients(List<IngredientJDO> ingredients) { this.ingredients.clear(); this.ingredients.addAll(ingredients); } </code></pre> <p>When you fetch the RecipeJDO, the <code>ingredients</code> list is not a real <code>ArrayList</code>, it is a dynamic proxy that handles the persistence of the contained elements. You shouldn't replace it.</p> <p>While the persistence manager is open, you can iterate through the <code>ingredients</code> list, add items or remove items, and the changes will be persisted when the persistence manager is closed (or the transaction is committed, if you are in a transaction). Here's how you would do the update without a transaction:</p> <pre><code>public void updateRecipe(String id, List<IngredientDTO> newIngredients) { List<IngredientJDO> ingredients = convertIngredientDtosToJdos(newIngredients); PersistenceManager pm = PMF.get().getPersistenceManager(); try { RecipeJDO recipe = pm.getObjectById(RecipeJDO.class, id); recipe.setIngredients(ingredients); } finally { pm.close(); } } </code></pre> <p>If you never modify the <code>IngredientJDO</code> objects (only replace them and read them), you might want to make them <code>Serializable</code> objects instead of JDO objects. If you do that, you may be able to reuse the <code>Ingredient</code> class in your GWT RPC code.</p> <p>Incidentally, even if <code>Recipe</code> was not a JDO object, you would want to make a copy in the <code>setIngredients()</code> method, otherwise someone could do this:</p> <pre><code>List<IngredientJDO> ingredients = new ArrayList<IngredientJDO>; // add items to ingredients recipe.setIngredients(ingredients); ingredients.clear(); // Woops! Modifies Recipe! </code></pre>

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