hibernate-012: Which Direction to Use for Each Relationship Type in Hibernate?

Which Direction to Use for Each Relationship Type in Hibernate?

Choosing unidirectional or bidirectional depends on:

How you query data.
Performance considerations.
Database design (avoiding unnecessary tables).

1️⃣ One-to-Many & Many-to-One

Relationship Type	Recommended Direction	Why?
One-to-Many (Unidirectional)	Not recommended	Creates an unnecessary join table.
One-to-Many (Bidirectional)	Recommended	Avoids a join table, allows querying in both directions.
Many-to-One (Unidirectional)	Recommended	Simpler, efficient for querying child-to-parent.

Best Practice:

Always use @ManyToOne with @JoinColumn (this is the owning side).
Use @OneToMany(mappedBy = "field") on the inverse side only if needed.

Example


<span>@Entity</span>
<span>public</span> <span>class</span> <span>Employee</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 stored in Employee</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>@OneToMany</span><span>(</span><span>mappedBy</span> <span>=</span> <span>"department"</span><span>)</span> <span>//  Inverse side</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>@ManyToOne</span>
    <span>@JoinColumn</span><span>(</span><span>name</span> <span>=</span> <span>"department_id"</span><span>)</span> <span>//  Foreign key stored in Employee</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>@OneToMany</span><span>(</span><span>mappedBy</span> <span>=</span> <span>"department"</span><span>)</span> <span>//  Inverse side</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 {
    @ManyToOne
    @JoinColumn(name = "department_id") //  Foreign key stored in Employee
    private Department department;
}

@Entity
public class Department {
    @OneToMany(mappedBy = "department") //  Inverse side
    private List<Employee> employees;
}

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Why?

Using only @ManyToOne is efficient (no extra join table).

Using bidirectional makes sense if you need to access employees from a department.

2️⃣ One-to-One

Relationship Type	Recommended Direction	Why?
One-to-One (Unidirectional)	Recommended	Simple, only one entity holds the foreign key.
One-to-One (Bidirectional)	Use if needed	Needed if both entities must reference each other.

Best Practice:

Use Unidirectional @OneToOne unless both sides must reference each other.
If using Bidirectional @OneToOne, ensure the inverse side has mappedBy.

Example


<span>@Entity</span>
<span>public</span> <span>class</span> <span>User</span> <span>{</span>
    <span>@OneToOne</span>
    <span>@JoinColumn</span><span>(</span><span>name</span> <span>=</span> <span>"profile_id"</span><span>)</span> <span>//  Foreign key in User</span>
    <span>private</span> <span>Profile</span> <span>profile</span><span>;</span>
<span>}</span>
<span>@Entity</span>
<span>public</span> <span>class</span> <span>Profile</span> <span>{</span>
    <span>@OneToOne</span><span>(</span><span>mappedBy</span> <span>=</span> <span>"profile"</span><span>)</span> <span>//  Inverse side</span>
    <span>private</span> <span>User</span> <span>user</span><span>;</span>
<span>}</span>
<span>@Entity</span>
<span>public</span> <span>class</span> <span>User</span> <span>{</span>
    <span>@OneToOne</span>
    <span>@JoinColumn</span><span>(</span><span>name</span> <span>=</span> <span>"profile_id"</span><span>)</span> <span>//  Foreign key in User</span>
    <span>private</span> <span>Profile</span> <span>profile</span><span>;</span>
<span>}</span>

<span>@Entity</span>
<span>public</span> <span>class</span> <span>Profile</span> <span>{</span>
    <span>@OneToOne</span><span>(</span><span>mappedBy</span> <span>=</span> <span>"profile"</span><span>)</span> <span>//  Inverse side</span>
    <span>private</span> <span>User</span> <span>user</span><span>;</span>
<span>}</span>
@Entity
public class User {
    @OneToOne
    @JoinColumn(name = "profile_id") //  Foreign key in User
    private Profile profile;
}

@Entity
public class Profile {
    @OneToOne(mappedBy = "profile") //  Inverse side
    private User user;
}

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Why?

Unidirectional is simple and avoids redundancy.

Bidirectional is useful if you frequently query both ways.

3️⃣ Many-to-Many

Relationship Type	Recommended Direction	Why?
Many-to-Many (Unidirectional)	Okay for simple use cases	One side owns the join table, but the other cannot query back.
Many-to-Many (Bidirectional)	Recommended	Both entities can query each other efficiently.

Best Practice:

Use bidirectional @ManyToMany to allow querying both sides.
Place @JoinTable on the owning side, use mappedBy on the inverse side.

Example


<span>@Entity</span>
<span>public</span> <span>class</span> <span>Student</span> <span>{</span>
    <span>@ManyToMany</span>
    <span>@JoinTable</span><span>(</span>
        <span>name</span> <span>=</span> <span>"student_course"</span><span>,</span>
        <span>joinColumns</span> <span>=</span> <span>@JoinColumn</span><span>(</span><span>name</span> <span>=</span> <span>"student_id"</span><span>),</span>
        <span>inverseJoinColumns</span> <span>=</span> <span>@JoinColumn</span><span>(</span><span>name</span> <span>=</span> <span>"course_id"</span><span>)</span>
    <span>)</span>
    <span>private</span> <span>List</span><span><</span><span>Course</span><span>></span> <span>courses</span><span>;</span>
<span>}</span>
<span>@Entity</span>
<span>public</span> <span>class</span> <span>Course</span> <span>{</span>
    <span>@ManyToMany</span><span>(</span><span>mappedBy</span> <span>=</span> <span>"courses"</span><span>)</span> <span>//  Inverse side</span>
    <span>private</span> <span>List</span><span><</span><span>Student</span><span>></span> <span>students</span><span>;</span>
<span>}</span>
<span>@Entity</span>
<span>public</span> <span>class</span> <span>Student</span> <span>{</span>
    <span>@ManyToMany</span>
    <span>@JoinTable</span><span>(</span>
        <span>name</span> <span>=</span> <span>"student_course"</span><span>,</span>
        <span>joinColumns</span> <span>=</span> <span>@JoinColumn</span><span>(</span><span>name</span> <span>=</span> <span>"student_id"</span><span>),</span>
        <span>inverseJoinColumns</span> <span>=</span> <span>@JoinColumn</span><span>(</span><span>name</span> <span>=</span> <span>"course_id"</span><span>)</span>
    <span>)</span>
    <span>private</span> <span>List</span><span><</span><span>Course</span><span>></span> <span>courses</span><span>;</span>
<span>}</span>

<span>@Entity</span>
<span>public</span> <span>class</span> <span>Course</span> <span>{</span>
    <span>@ManyToMany</span><span>(</span><span>mappedBy</span> <span>=</span> <span>"courses"</span><span>)</span> <span>//  Inverse side</span>
    <span>private</span> <span>List</span><span><</span><span>Student</span><span>></span> <span>students</span><span>;</span>
<span>}</span>
@Entity
public class Student {
    @ManyToMany
    @JoinTable(
        name = "student_course",
        joinColumns = @JoinColumn(name = "student_id"),
        inverseJoinColumns = @JoinColumn(name = "course_id")
    )
    private List<Course> courses;
}

@Entity
public class Course {
    @ManyToMany(mappedBy = "courses") //  Inverse side
    private List<Student> students;
}

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Why?

Unidirectional works, but bidirectional is better for querying both ways.

Avoids duplicate join tables.

Final Recommendations

Relationship	Best Approach	Why?
One-to-Many & Many-to-One	Use `@ManyToOne` + `@OneToMany(mappedBy)`	Efficient, avoids extra join tables.
One-to-One	Use Unidirectional `@OneToOne`, bidirectional only if necessary	Simple and avoids redundancy.
Many-to-Many	Use Bidirectional `@ManyToMany`	Allows querying both ways without duplicating join tables.

Best Practice: Keep it simple. Use bidirectional only if both sides need to query each other.

原文链接：hibernate-012: Which Direction to Use for Each Relationship Type in Hibernate?

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