How to map an ArrayList of primitives to a single column?

问题

Let's say I have the following situation:

Object Car has an ArrayList of prices, which are all numbers. Is it possible in Hibernate to save all the prices in a single column? I know this violates the first normal form but there might be cases when you don't want them to be saved in a separate table like it's classically done in One-To-Many or Many-To-Many relationships.

In JDO I'd do this easily by saving the ArrayList in a BLOB column.

Some useful related SOF questions: ArrayList of primitive types in Hibernate and Map ArrayList with Hibernate .

Any idea will be highly appreciated!

回答1:

I know this is an old question but for anyone trying to do this in a JPA context you can do this

import org.apache.commons.lang3.StringUtils;
import javax.persistence.AttributeConverter;
import javax.persistence.Converter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import java.util.Collections;

@Converter
public class IntArrayToStringConverter implements AttributeConverter<List<Integer>,String>{
    @Override
    public String convertToDatabaseColumn(List<Integer> attribute) {
        return attribute == null ? null : StringUtils.join(attribute,",");
    }

    @Override
    public List<Integer> convertToEntityAttribute(String dbData) {
        if (StringUtils.isBlank(dbData))
            return Collections.emptyList();

        try (Stream<String> stream = Arrays.stream(dbData.split(","))) {
            return stream.map(Integer::parseInt).collect(Collectors.toList());
        }
    }
}

Then to use it something like this in your entity

@Entity
public class SomeEntity
{

   @Id
   @GeneratedValue(strategy = GenerationType.IDENTITY)
   private Integer id;

   @Column
   @Convert(converter = IntArrayToStringConverter.class)
   private List<Integer> integers;

   ...
}

回答2:

AFAIR, Hibernate will use native serialization and store the resulting bytes in your column. I wouldn't do that though, and use a dedicated transformation that would make the prices readable in the database, and at least be able to use the data without needing Java native serialization:

@Basic
private String prices;

public void setPrices(List<Integer> prices) {
    this.prices = Joiner.on(',').join(prices);
}

public List<Integer> getPrices() {
    List<Integer> result = new ArrayList<Integer>();
    for (String s : Splitter.on(',').split(this.prices)) {
        result.add(Integer.valueOf(s));
    }
    return result;
}

回答3:

You can implement your own custom type as an array:

http://docs.jboss.org/hibernate/core/3.6/reference/en-US/html/types.html#types-custom

Also, it is not that hard to find some implementations, some of them going as far to let you compare those arrays in a HQL where clause.

https://forum.hibernate.org/viewtopic.php?t=946973

I personally never thought i would try something like this. But now I am very curious.

来源：https://stackoverflow.com/questions/16373811/how-to-map-an-arraylist-of-primitives-to-a-single-column

标签

hibernate

many-to-many

one-to-many