问题
If i have several entites, lets say :
@Entity
class Book{
String name;
Author author;
}
@Entity
class Author{
String name;
City hometown;
}
@Entity
class City{
String cityName;
}
If i want to retrieve all the books, if i use classic JPA Repository and Spring Data and just do a findAll(), it will get me all the books with all the Authors with all their home towns. I know i can use @JsonIgnore, but i think that only prevents whats being returned, not whats being looked up in the database. And also i have methods that DO want to return both books and authors, so @JsonIgnore -ing does not work for me. Is there anything like this ? To tell Spring Data what to look up and what to return ? Any links or guides or methods i don't know of, would be appreciated.
回答1:
Spring Data has the concept of 'projections' which allow you to return different representations of the same Entity.
Official Documentation:
Spring Data query methods usually return one or multiple instances of the aggregate root managed by the repository. However, it might sometimes be desirable to create projections based on certain attributes of those types. Spring Data allows modeling dedicated return types, to more selectively retrieve partial views of the managed aggregates.
https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#projections
Where a Projection is a 'closed' projection (a projection interface whose accessor methods all match properties of the target aggregate) then the documentation notes that additionally:
Spring Data can optimize the query execution [to select only the relevant fields], because we know about all the attributes that are needed to back the projection proxy
https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#projections.interfaces.closed
Spring Data also allows for Projections to be specified dynamically at runtime. See further:
https://github.com/spring-projects/spring-data-commons/blob/master/src/main/asciidoc/repository-projections.adoc#dynamic-projections
回答2:
- First mark your relations as LAZY
- Then specify what data needs to be fetched on a per-query basis.
See for example: https://vladmihalcea.com/eager-fetching-is-a-code-smell/
来源：https://stackoverflow.com/questions/56130098/spring-data-how-to-tell-spring-what-entities-to-retrieve