阅读golang ent relation文档

Golang/ent.md

@@ -75,3 +75,165 @@ ent
 └── user_update.go
 ```
 
+### 创建表并插入实体
+#### 创建表
+在执行完上述步骤后，可以通过如下代码来创建table
+```golang
+package main
+
+import (
+    "context"
+    "log"
+
+    "entdemo/ent"
+
+    _ "github.com/go-sql-driver/mysql"
+)
+
+func main() {
+    client, err := ent.Open("mysql", "<user>:<pass>@tcp(<host>:<port>)/<database>?parseTime=True")
+    if err != nil {
+        log.Fatalf("failed opening connection to mysql: %v", err)
+    }
+    defer client.Close()
+    // Run the auto migration tool.
+    if err := client.Schema.Create(context.Background()); err != nil {
+        log.Fatalf("failed creating schema resources: %v", err)
+    }
+}
+```
+> 在执行上述代码前，需要先通过`create database xxx`先创建数据库。
+
+#### 插入实体
+通过如下代码，可以插入实体：
+```golang
+entdemo/start.go
+func CreateUser(ctx context.Context, client *ent.Client) (*ent.User, error) {
+    u, err := client.User.
+        Create().
+        SetAge(30).
+        SetName("a8m").
+        Save(ctx)
+    if err != nil {
+        return nil, fmt.Errorf("failed creating user: %w", err)
+    }
+    log.Println("user was created: ", u)
+    return u, nil
+}
+```
+
+### 查询实体
+查询实体可以通过如下代码实现：
+```golang
+func QueryUser(ctx context.Context, client *ent.Client) (ul []*ent.User, err error) {
+	ul, err = client.User.
+		Query().
+		Where(
+			user.Age(30),
+			user.Name("a8m"),
+		).All(ctx)
+	if err != nil {
+		log.Errorf("failed querying user: %v", err)
+	}
+	return
+}
+```
+
+### 添加关联关系
+另外创建两个实体`Car`和`Group`
+```bash
+go run -mod=mod entgo.io/ent/cmd/ent new Car Group
+```
+修改Car实体如下
+```golang
+package schema
+
+import (
+	"entgo.io/ent"
+	"entgo.io/ent/schema/field"
+)
+
+// Car holds the schema definition for the Car entity.
+type Car struct {
+	ent.Schema
+}
+
+// Fields of the Car.
+func (Car) Fields() []ent.Field {
+	return []ent.Field{
+		field.String("model"),
+		field.Time("registered_at"),
+	}
+}
+
+// Edges of the Car.
+func (Car) Edges() []ent.Edge {
+	return nil
+}
+```
+修改Group实体如下：
+```golang
+package schema
+
+import (
+	"entgo.io/ent"
+	"entgo.io/ent/schema/field"
+	"regexp"
+)
+
+// Group holds the schema definition for the Group entity.
+type Group struct {
+	ent.Schema
+}
+
+// Fields of the Group.
+func (Group) Fields() []ent.Field {
+	return []ent.Field{
+		field.String("name").
+			// Regexp validation for group name.
+			Match(regexp.MustCompile("[a-zA-Z_]+$")),
+	}
+}
+
+// Edges of the Group.
+func (Group) Edges() []ent.Edge {
+	return nil
+}
+```
+并修改User实体如下，为其新增`Edges`方法
+```golang
+package schema
+
+import (
+	"entgo.io/ent"
+	"entgo.io/ent/schema/edge"
+	"entgo.io/ent/schema/field"
+)
+
+// User holds the schema definition for the User entity.
+type User struct {
+	ent.Schema
+}
+
+// Fields of the User.
+func (User) Fields() []ent.Field {
+	return []ent.Field{
+		field.Int("age").Positive(),
+		field.String("name").Default("unknown"),
+	}
+}
+
+// Edges of the User.
+func (User) Edges() []ent.Edge {
+	return []ent.Edge{
+		edge.To("cars", Car.Type),
+	}
+}
+```
+可知，Car和User实体的关系如下图所示：
+- 一个user可以拥有多个car
+
+<img loading="lazy" src="https://entgo.io/images/assets/re_user_cars.png" alt="er-user-cars" class="img_ev3q" id="er-user-cars">
+
+运行`go generate ./ent`后如下：
+```bash