## Why Does Hibernate Criteria with FetchType.EAGER Result in Duplicate Order Objects?

Hibernate Criteria Returns Multiple Repeated Orders with FetchType.EAGER

Problem:

When using a Hibernate Criteria with FetchType.EAGER for a one-to-many relationship, the resulting list contains multiple copies of the same Order object.

Original Code:

<code class="java">public List<OrderTransaction> getOrderTransactions() {
    return orderTransactions;
}</code>

Modified Code with FetchType.EAGER:

<code class="java">public List<OrderTransaction> getOrderTransactions() {
    return orderTransactions;
}</code>

Question:

Why does the modified code with FetchType.EAGER result in multiple copies of the Order object?

Answer:

This behavior is expected when enabling eager fetching. A join is performed between the Order and OrderTransaction tables, resulting in the same number of rows as a regular SQL join. Therefore, each Order object appears multiple times, corresponding to the number of related OrderTransaction records.

Explanation:

An outer join is used when fetching data with FetchType.EAGER. This means that even if an Order has no related OrderTransaction records, it will still be included in the result set. The result is a list of Order objects, each potentially associated with multiple OrderTransaction objects.

Solution:

To avoid multiple copies of the Order object, consider using a set instead of a list to prevent duplicates. Alternatively, use a DistinctRootEntity in the Criteria query, which explicitly specifies the distinct Order objects to be returned.

Example:

<code class="java">Criteria criteria = getHibernateSession()
        .createCriteria(Order.class)
        .setResultTransformer(CriteriaSpecification.DISTINCT_ROOT_ENTITY)
        .add(Restrictions.in("orderStatus", orderFilter.getStatusesToShow()));</code>

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