基于GORM实现CreateOrUpdate方法详解

// Create insert the value into database
func (db *DB) Create(value interface{}) (tx *DB) {
	if db.CreateBatchSize > 0 {
		return db.CreateInBatches(value, db.CreateBatchSize)
	}
	tx = db.getInstance()
	tx.Statement.Dest = value
	return tx.callbacks.Create().Execute(tx)
}

// Save update value in database, if the value doesn't have primary key, will insert it
func (db *DB) Save(value interface{}) (tx *DB) {
	tx = db.getInstance()
	tx.Statement.Dest = value
	reflectValue := reflect.Indirect(reflect.ValueOf(value))
	for reflectValue.Kind() == reflect.Ptr || reflectValue.Kind() == reflect.Interface {
		reflectValue = reflect.Indirect(reflectValue)
	}
	switch reflectValue.Kind() {
	case reflect.Slice, reflect.Array:
		if _, ok := tx.Statement.Clauses["ON CONFLICT"]; !ok {
			tx = tx.Clauses(clause.OnConflict{UpdateAll: true})
		}
		tx = tx.callbacks.Create().Execute(tx.Set("gorm:update_track_time", true))
	case reflect.Struct:
		if err := tx.Statement.Parse(value); err == nil && tx.Statement.Schema != nil {
			for _, pf := range tx.Statement.Schema.PrimaryFields {
				if _, isZero := pf.ValueOf(tx.Statement.Context, reflectValue); isZero {
					return tx.callbacks.Create().Execute(tx)
				}
			}
		}
		fallthrough
	default:
		selectedUpdate := len(tx.Statement.Selects) != 0
		// when updating, use all fields including those zero-value fields
		if !selectedUpdate {
			tx.Statement.Selects = append(tx.Statement.Selects, "*")
		}
		tx = tx.callbacks.Update().Execute(tx)
		if tx.Error == nil && tx.RowsAffected == 0 && !tx.DryRun && !selectedUpdate {
			result := reflect.New(tx.Statement.Schema.ModelType).Interface()
			if result := tx.Session(&Session{}).Limit(1).Find(result); result.RowsAffected == 0 {
				return tx.Create(value)
			}
		}
	}
	return
}

// Update update attributes with callbacks, refer: https://gorm.io/docs/update.html#Update-Changed-Fields
func (db *DB) Update(column string, value interface{}) (tx *DB) {
	tx = db.getInstance()
	tx.Statement.Dest = map[string]interface{}{column: value}
	return tx.callbacks.Update().Execute(tx)
}
// Updates update attributes with callbacks, refer: https://gorm.io/docs/update.html#Update-Changed-Fields
func (db *DB) Updates(values interface{}) (tx *DB) {
	tx = db.getInstance()
	tx.Statement.Dest = values
	return tx.callbacks.Update().Execute(tx)
}

// FirstOrInit gets the first matched record or initialize a new instance with given conditions (only works with struct or map conditions)
func (db *DB) FirstOrInit(dest interface{}, conds ...interface{}) (tx *DB) {
	queryTx := db.Limit(1).Order(clause.OrderByColumn{
		Column: clause.Column{Table: clause.CurrentTable, Name: clause.PrimaryKey},
	})
	if tx = queryTx.Find(dest, conds...); tx.RowsAffected == 0 {
		if c, ok := tx.Statement.Clauses["WHERE"]; ok {
			if where, ok := c.Expression.(clause.Where); ok {
				tx.assignInterfacesToValue(where.Exprs)
			}
		}
		// initialize with attrs, conds
		if len(tx.Statement.attrs) > 0 {
			tx.assignInterfacesToValue(tx.Statement.attrs...)
		}
	}
	// initialize with attrs, conds
	if len(tx.Statement.assigns) > 0 {
		tx.assignInterfacesToValue(tx.Statement.assigns...)
	}
	return
}

// FirstOrCreate gets the first matched record or create a new one with given conditions (only works with struct, map conditions)
func (db *DB) FirstOrCreate(dest interface{}, conds ...interface{}) (tx *DB) {
	tx = db.getInstance()
	queryTx := db.Session(&Session{}).Limit(1).Order(clause.OrderByColumn{
		Column: clause.Column{Table: clause.CurrentTable, Name: clause.PrimaryKey},
	})
	if result := queryTx.Find(dest, conds...); result.Error == nil {
		if result.RowsAffected == 0 {
			if c, ok := result.Statement.Clauses["WHERE"]; ok {
				if where, ok := c.Expression.(clause.Where); ok {
					result.assignInterfacesToValue(where.Exprs)
				}
			}
			// initialize with attrs, conds
			if len(db.Statement.attrs) > 0 {
				result.assignInterfacesToValue(db.Statement.attrs...)
			}
			// initialize with attrs, conds
			if len(db.Statement.assigns) > 0 {
				result.assignInterfacesToValue(db.Statement.assigns...)
			}
			return tx.Create(dest)
		} else if len(db.Statement.assigns) > 0 {
			exprs := tx.Statement.BuildCondition(db.Statement.assigns[0], db.Statement.assigns[1:]...)
			assigns := map[string]interface{}{}
			for _, expr := range exprs {
				if eq, ok := expr.(clause.Eq); ok {
					switch column := eq.Column.(type) {
					case string:
						assigns[column] = eq.Value
					case clause.Column:
						assigns[column.Name] = eq.Value
					default:
					}
				}
			}
			return tx.Model(dest).Updates(assigns)
		}
	} else {
		tx.Error = result.Error
	}
	return tx
}

func (db *DB) Attrs(attrs ...interface{}) (tx *DB) {
	tx = db.getInstance()
	tx.Statement.attrs = attrs
	return
}
func (db *DB) Assign(attrs ...interface{}) (tx *DB) {
	tx = db.getInstance()
	tx.Statement.assigns = attrs
	return
}

// 未找到 user，根据条件和 Assign 属性创建记录
db.Where(User{Name: "non_existing"}).Assign(User{Age: 20}).FirstOrCreate(&user)
// SELECT * FROM users WHERE name = 'non_existing' ORDER BY id LIMIT 1;
// INSERT INTO "users" (name, age) VALUES ("non_existing", 20);
// user -> User{ID: 112, Name: "non_existing", Age: 20}
// 找到了 `name` = `jinzhu` 的 user，依然会根据 Assign 更新记录
db.Where(User{Name: "jinzhu"}).Assign(User{Age: 20}).FirstOrCreate(&user)
// SELECT * FROM users WHERE name = 'jinzhu' ORDER BY id LIMIT 1;
// UPDATE users SET age=20 WHERE id = 111;
// user -> User{ID: 111, Name: "jinzhu", Age: 20}

import "gorm.io/gorm/clause"
// 不处理冲突
DB.Clauses(clause.OnConflict{DoNothing: true}).Create(&user)
// `id` 冲突时，将字段值更新为默认值
DB.Clauses(clause.OnConflict{
  Columns:   []clause.Column{{Name: "id"}},
  DoUpdates: clause.Assignments(map[string]interface{}{"role": "user"}),
}).Create(&users)
// MERGE INTO "users" USING *** WHEN NOT MATCHED THEN INSERT *** WHEN MATCHED THEN UPDATE SET ***; SQL Server
// INSERT INTO `users` *** ON DUPLICATE KEY UPDATE ***; MySQL
// Update columns to new value on `id` conflict
DB.Clauses(clause.OnConflict{
  Columns:   []clause.Column{{Name: "id"}},
  DoUpdates: clause.AssignmentColumns([]string{"name", "age"}),
}).Create(&users)
// MERGE INTO "users" USING *** WHEN NOT MATCHED THEN INSERT *** WHEN MATCHED THEN UPDATE SET "name"="excluded"."name"; SQL Server
// INSERT INTO "users" *** ON CONFLICT ("id") DO UPDATE SET "name"="excluded"."name", "age"="excluded"."age"; PostgreSQL
// INSERT INTO `users` *** ON DUPLICATE KEY UPDATE `name`=VALUES(name),`age=VALUES(age); MySQL

type Interface interface {  
    Name() string  
    Build(Builder)  
    MergeClause(*Clause)  
}
// AddClause add clause
func (stmt *Statement) AddClause(v clause.Interface) {
	if optimizer, ok := v.(StatementModifier); ok {
		optimizer.ModifyStatement(stmt)
	} else {
		name := v.Name()
		c := stmt.Clauses[name]
		c.Name = name
		v.MergeClause(&c)
		stmt.Clauses[name] = c
	}
}

package clause
type OnConflict struct {
	Columns      []Column
	Where        Where
	TargetWhere  Where
	OnConstraint string
	DoNothing    bool
	DoUpdates    Set
	UpdateAll    bool
}
func (OnConflict) Name() string {
	return "ON CONFLICT"
}
// Build build onConflict clause
func (onConflict OnConflict) Build(builder Builder) {
	if len(onConflict.Columns) > 0 {
		builder.WriteByte('(')
		for idx, column := range onConflict.Columns {
			if idx > 0 {
				builder.WriteByte(',')
			}
			builder.WriteQuoted(column)
		}
		builder.WriteString(`) `)
	}
	if len(onConflict.TargetWhere.Exprs) > 0 {
		builder.WriteString(" WHERE ")
		onConflict.TargetWhere.Build(builder)
		builder.WriteByte(' ')
	}
	if onConflict.OnConstraint != "" {
		builder.WriteString("ON CONSTRAINT ")
		builder.WriteString(onConflict.OnConstraint)
		builder.WriteByte(' ')
	}
	if onConflict.DoNothing {
		builder.WriteString("DO NOTHING")
	} else {
		builder.WriteString("DO UPDATE SET ")
		onConflict.DoUpdates.Build(builder)
	}
	if len(onConflict.Where.Exprs) > 0 {
		builder.WriteString(" WHERE ")
		onConflict.Where.Build(builder)
		builder.WriteByte(' ')
	}
}
// MergeClause merge onConflict clauses
func (onConflict OnConflict) MergeClause(clause *Clause) {
	clause.Expression = onConflict
}

type Set []Assignment
type Assignment struct {
	Column Column
	Value  interface{}
}