Enum RelationKind 

pub enum RelationKind {
    HasOne,
    HasMany,
    BelongsTo,
    ManyToMany,
}

关联关系类型

决定 JOIN 策略和数据加载方式：

• HasOne / BelongsTo → INNER JOIN（一条关联记录）
• HasMany / ManyToMany → LEFT JOIN（多条关联记录，双查询策略避免行膨胀）

Variants

HasOne

一对一：当前实体拥有一个关联实体（如 User → Profile）

HasMany

一对多：当前实体拥有多个关联实体（如 User → Orders）

BelongsTo

多对一：当前实体属于一个父实体（如 Order → User）

ManyToMany

多对多：通过中间表关联（如 User ↔ Role，通过 user_roles）

Implementations

impl RelationKind

pub fn default_join_type(self) -> JoinKind

返回该关系类型默认的 JOIN 类型

• HasOne / BelongsTo → JoinKind::Inner（关联记录存在性要求）
• HasMany / ManyToMany → JoinKind::Left（允许零关联记录）

Trait Implementations

impl Clone for RelationKind

fn clone(&self) -> RelationKind

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for RelationKind

impl Debug for RelationKind

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for RelationKind

impl Hash for RelationKind

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl PartialEq for RelationKind

fn eq(&self, other: &RelationKind) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for RelationKind