16.2.5. Returning Multiple Entities

Until now, the result set column names are assumed to be the same as the column names specified in the mapping document. This can be problematic for SQL queries that join multiple tables, since the same column names can appear in more than one table.

Column alias injection is needed in the following query (which most likely will fail):

sess.createSQLQuery("SELECT c.*, m.*  FROM CATS c, CATS m WHERE c.MOTHER_ID = c.ID")
 .addEntity("cat", Cat.class)
 .addEntity("mother", Cat.class)

The query was intended to return two Cat instances per row: a cat and its mother. The query will, however, fail because there is a conflict of names; the instances are mapped to the same column names. Also, on some databases the returned column aliases will most likely be on the form "c.ID", "c.NAME", etc. which are not equal to the columns specified in the mappings ("ID" and "NAME").

The following form is not vulnerable to column name duplication:

sess.createSQLQuery("SELECT {cat.*}, {mother.*}  FROM CATS c, CATS m WHERE c.MOTHER_ID = c.ID")
 .addEntity("cat", Cat.class)
 .addEntity("mother", Cat.class)

This query specified:

the SQL query string, with placeholders for Hibernate to inject column aliases
the entities returned by the query

The {cat.*} and {mother.*} notation used above is a shorthand for "all properties". Alternatively, you can list the columns explicitly, but even in this case Hibernate injects the SQL column aliases for each property. The placeholder for a column alias is just the property name qualified by the table alias. In the following example, you retrieve Cats and their mothers from a different table (cat_log) to the one declared in the mapping metadata. You can even use the property aliases in the where clause.

String sql = "SELECT ID as {c.id}, NAME as {c.name}, " + 
         "BIRTHDATE as {c.birthDate}, MOTHER_ID as {c.mother}, {mother.*} " +
         "FROM CAT_LOG c, CAT_LOG m WHERE {c.mother} = c.ID";

List loggedCats = sess.createSQLQuery(sql)
        .addEntity("cat", Cat.class)
        .addEntity("mother", Cat.class).list()

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16.2.5. Returning Multiple Entities

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