hibernate-013: Can You Mix Unidirectional and Bidirectional @OneToMany & @ManyToOne?

Can You Mix Unidirectional and Bidirectional `@OneToMany` & `@ManyToOne`?

Yes, you can! You can choose:

@ManyToOne without @OneToMany → Unidirectional
@ManyToOne with @OneToMany(mappedBy) → Bidirectional

Mixing Unidirectional & Bidirectional Approaches

Scenario	`@ManyToOne` Used?	`@OneToMany` Used?	Direction Type
Best Practice	Yes	Yes (`mappedBy`)	Bidirectional
Alternative	Yes	No	Unidirectional (`@ManyToOne` only)

1️⃣ Example: `@ManyToOne` Without `@OneToMany` (Unidirectional)

Best for queries from Employee → Department but NOT the other way around.


<span>@Entity</span>
<span>public</span> <span>class</span> <span>Employee</span> <span>{</span>
    <span>@Id</span>
    <span>@GeneratedValue</span><span>(</span><span>strategy</span> <span>=</span> <span>GenerationType</span><span>.</span><span>IDENTITY</span><span>)</span>
    <span>private</span> <span>Long</span> <span>id</span><span>;</span>
    <span>private</span> <span>String</span> <span>name</span><span>;</span>
    <span>@ManyToOne</span>
    <span>@JoinColumn</span><span>(</span><span>name</span> <span>=</span> <span>"department_id"</span><span>)</span> <span>//  Foreign key in Employee table</span>
    <span>private</span> <span>Department</span> <span>department</span><span>;</span>
<span>}</span>
<span>@Entity</span>
<span>public</span> <span>class</span> <span>Department</span> <span>{</span>
    <span>@Id</span>
    <span>@GeneratedValue</span><span>(</span><span>strategy</span> <span>=</span> <span>GenerationType</span><span>.</span><span>IDENTITY</span><span>)</span>
    <span>private</span> <span>Long</span> <span>id</span><span>;</span>
    <span>private</span> <span>String</span> <span>name</span><span>;</span>
<span>}</span>
<span>@Entity</span>
<span>public</span> <span>class</span> <span>Employee</span> <span>{</span>
    <span>@Id</span>
    <span>@GeneratedValue</span><span>(</span><span>strategy</span> <span>=</span> <span>GenerationType</span><span>.</span><span>IDENTITY</span><span>)</span>
    <span>private</span> <span>Long</span> <span>id</span><span>;</span>
    <span>private</span> <span>String</span> <span>name</span><span>;</span>

    <span>@ManyToOne</span>
    <span>@JoinColumn</span><span>(</span><span>name</span> <span>=</span> <span>"department_id"</span><span>)</span> <span>//  Foreign key in Employee table</span>
    <span>private</span> <span>Department</span> <span>department</span><span>;</span>
<span>}</span>

<span>@Entity</span>
<span>public</span> <span>class</span> <span>Department</span> <span>{</span>
    <span>@Id</span>
    <span>@GeneratedValue</span><span>(</span><span>strategy</span> <span>=</span> <span>GenerationType</span><span>.</span><span>IDENTITY</span><span>)</span>
    <span>private</span> <span>Long</span> <span>id</span><span>;</span>
    <span>private</span> <span>String</span> <span>name</span><span>;</span>
<span>}</span>
@Entity
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    @ManyToOne
    @JoinColumn(name = "department_id") //  Foreign key in Employee table
    private Department department;
}

@Entity
public class Department {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
}

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Queries:

employee.getDepartment(); (Works )
department.getEmployees(); (Not possible – No @OneToMany)

Use case: If you only need to retrieve an employee’s department, but NOT employees from a department.

2️⃣ Example: `@ManyToOne` + `@OneToMany(mappedBy)` (Bidirectional)

Best if you need queries in both directions.


<span>@Entity</span>
<span>public</span> <span>class</span> <span>Employee</span> <span>{</span>
    <span>@Id</span>
    <span>@GeneratedValue</span><span>(</span><span>strategy</span> <span>=</span> <span>GenerationType</span><span>.</span><span>IDENTITY</span><span>)</span>
    <span>private</span> <span>Long</span> <span>id</span><span>;</span>
    <span>private</span> <span>String</span> <span>name</span><span>;</span>
    <span>@ManyToOne</span>
    <span>@JoinColumn</span><span>(</span><span>name</span> <span>=</span> <span>"department_id"</span><span>)</span> <span>//  Foreign key in Employee table</span>
    <span>private</span> <span>Department</span> <span>department</span><span>;</span>
<span>}</span>
<span>@Entity</span>
<span>public</span> <span>class</span> <span>Department</span> <span>{</span>
    <span>@Id</span>
    <span>@GeneratedValue</span><span>(</span><span>strategy</span> <span>=</span> <span>GenerationType</span><span>.</span><span>IDENTITY</span><span>)</span>
    <span>private</span> <span>Long</span> <span>id</span><span>;</span>
    <span>private</span> <span>String</span> <span>name</span><span>;</span>
    <span>@OneToMany</span><span>(</span><span>mappedBy</span> <span>=</span> <span>"department"</span><span>)</span> <span>//  Refers to Employee.department</span>
    <span>private</span> <span>List</span><span><</span><span>Employee</span><span>></span> <span>employees</span><span>;</span>
<span>}</span>
<span>@Entity</span>
<span>public</span> <span>class</span> <span>Employee</span> <span>{</span>
    <span>@Id</span>
    <span>@GeneratedValue</span><span>(</span><span>strategy</span> <span>=</span> <span>GenerationType</span><span>.</span><span>IDENTITY</span><span>)</span>
    <span>private</span> <span>Long</span> <span>id</span><span>;</span>
    <span>private</span> <span>String</span> <span>name</span><span>;</span>

    <span>@ManyToOne</span>
    <span>@JoinColumn</span><span>(</span><span>name</span> <span>=</span> <span>"department_id"</span><span>)</span> <span>//  Foreign key in Employee table</span>
    <span>private</span> <span>Department</span> <span>department</span><span>;</span>
<span>}</span>

<span>@Entity</span>
<span>public</span> <span>class</span> <span>Department</span> <span>{</span>
    <span>@Id</span>
    <span>@GeneratedValue</span><span>(</span><span>strategy</span> <span>=</span> <span>GenerationType</span><span>.</span><span>IDENTITY</span><span>)</span>
    <span>private</span> <span>Long</span> <span>id</span><span>;</span>
    <span>private</span> <span>String</span> <span>name</span><span>;</span>

    <span>@OneToMany</span><span>(</span><span>mappedBy</span> <span>=</span> <span>"department"</span><span>)</span> <span>//  Refers to Employee.department</span>
    <span>private</span> <span>List</span><span><</span><span>Employee</span><span>></span> <span>employees</span><span>;</span>
<span>}</span>
@Entity
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    @ManyToOne
    @JoinColumn(name = "department_id") //  Foreign key in Employee table
    private Department department;
}

@Entity
public class Department {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    @OneToMany(mappedBy = "department") //  Refers to Employee.department
    private List<Employee> employees;
}

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Queries:

employee.getDepartment(); (Works )
department.getEmployees(); (Works )

Use case: If you need to retrieve both:

All employees in a department
An employee’s department

Final Recommendation

When to Use	Use `@ManyToOne` Only (Unidirectional)	Use `@ManyToOne` + `@OneToMany` (Bidirectional)
Simple querying (best performance)	Yes	No
Only querying child → parent	Yes	No
Need parent → child queries	No	Yes
Avoiding unnecessary complexity	Yes	No

Best Practice:

Use @ManyToOne by itself (Unidirectional) if querying only from child → parent.
Use @ManyToOne + @OneToMany(mappedBy) (Bidirectional) if querying both ways.

原文链接：hibernate-013: Can You Mix Unidirectional and Bidirectional @OneToMany & @ManyToOne?

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