update readme

README.md

@@ -23,9 +23,63 @@ go get github.com/JunNishimura/casbin-bun-adapter
 
 ## 👀 Example
 ```go
+package main
 
+import (
+	casbinbunadapter "github.com/JunNishimura/casbin-bun-adapter"
+	"github.com/casbin/casbin/v2"
+)
+
+func main() {
+	// initialize a Bun adapter and use it in a Casbin enforcer
+	a, _ := casbinbunadapter.NewAdapter("mysql", "mysql_username:mysql_password@tcp(127.0.0.1:3306)/database")
+	e, _ := casbin.NewEnforcer("model.conf", a)
+
+	// load the policy from DB.
+	_ = e.LoadPolicy()
+
+	// check the permission.
+	_, _ = e.Enforce("alice", "data1", "read")
+
+	// modify the policy
+	// e.AddPolicy(...)
+	// e.RemovePolicy(...)
+	// e.UpdatePolicy(...)
+
+	// save the policy back to DB.
+	_ = e.SavePolicy()
+}
+```
+
+## 😢 Limitations
+casbin-bun-adapter has following limitations.
+### 1. Table names cannot be freely specified
+To specify the table name in Bun, you need to specify it in a structure tag or call the ModelTableExpr method in the query builder.
+```go
+type User struct {
+  bun.BaseModel `bun:"table:users,alias:u"`
+  ID    int64  `bun:"id,pk,autoincrement"`
+  Name  string `bun:"name,notnull"`
+}
 ```
 
+```go
+res, err := db.NewInsert().
+    Model(user).
+    ModelTableExpr("custom_name") // specify table name
+    Exec(ctx)
+```
+If you want to create a table with a name specified by the user, you can use ModelTableExpr, but I gave up using ModelTableExpr because I found that query build to tuncate table does not support ModelTableExpr.
+
+If we come up with a better approach, or if Bun's specifications regarding the above change, we will modify this one accordingly.
+
+## 2. Unique indexes cannot be added on columns in the casbin_policies table
+For Postgres, you can specify `IF NOT EXISTS` to create a key only when the key does not exist, but other DBs do not support the above syntax by default.
+
+There seems to be no way to check if the index is posted in Bun.
+
+If I find the way to check if the index exists with SQL, it might be possible to achieve this, so I'll leave this as a future issue.
+
 ## 🙇‍♂️ Thanks
 I would like to express my appreciation to [Gorm Adapter](https://github.com/casbin/gorm-adapter), since casbin-bun-adapter is implemented in a way that fits the Bun ORM based on it.