简介
GORM 源码解读, 基于 v1.9.11 版本.
查询
上一节中, 我们已经探究过了模型是如何定义的, 以及数据表是如何创建的.
这次, 看一下查询是如何实现的.
查询涉及到很大的一块内容, 因为要支持各种类型的方法.
先看一下官方文档中提供的最简单的几个查询方法.
// 根据主键查询第一条记录
db.First(&user)
//// SELECT * FROM users ORDER BY id LIMIT 1;
// 随机获取一条记录
db.Take(&user)
//// SELECT * FROM users LIMIT 1;
// 根据主键查询最后一条记录
db.Last(&user)
//// SELECT * FROM users ORDER BY id DESC LIMIT 1;
// 查询所有的记录
db.Find(&users)
//// SELECT * FROM users;
// 查询指定的某条记录(仅当主键为整型时可用)
db.First(&user, 10)
//// SELECT * FROM users WHERE id = 10;以 First 方法为例, 看一下它的实现:
// First find first record that match given conditions, order by primary key
func (s *DB) First(out interface{}, where ...interface{}) *DB {
newScope := s.NewScope(out)
newScope.Search.Limit(1)
return newScope.Set("gorm:order_by_primary_key", "ASC").
inlineCondition(where...).callCallbacks(s.parent.callbacks.queries).db
}First 方法从数据库中获取第一条数据, 以 primary key 升序排序.
前面介绍过, 具体的数据库操作实现是依靠 callbacks 的. 这里用到了 callbacks.queries.
在默认的 callbacks 中, 注册了三个不同的 query 回调函数.
// Define callbacks for querying
func init() {
DefaultCallback.Query().Register("gorm:query", queryCallback)
DefaultCallback.Query().Register("gorm:preload", preloadCallback)
DefaultCallback.Query().Register("gorm:after_query", afterQueryCallback)
}查询流程
先来看一下最主要的 queryCallback 函数.
// queryCallback used to query data from database
func queryCallback(scope *Scope) {
if _, skip := scope.InstanceGet("gorm:skip_query_callback"); skip {
return
}
//we are only preloading relations, dont touch base model
if _, skip := scope.InstanceGet("gorm:only_preload"); skip {
return
}
defer scope.trace(scope.db.nowFunc())
var (
isSlice, isPtr bool
resultType reflect.Type
results = scope.IndirectValue()
)
if orderBy, ok := scope.Get("gorm:order_by_primary_key"); ok {
if primaryField := scope.PrimaryField(); primaryField != nil {
scope.Search.Order(fmt.Sprintf("%v.%v %v", scope.QuotedTableName(), scope.Quote(primaryField.DBName), orderBy))
}
}
if value, ok := scope.Get("gorm:query_destination"); ok {
results = indirect(reflect.ValueOf(value))
}
if kind := results.Kind(); kind == reflect.Slice {
isSlice = true
resultType = results.Type().Elem()
results.Set(reflect.MakeSlice(results.Type(), 0, 0))
if resultType.Kind() == reflect.Ptr {
isPtr = true
resultType = resultType.Elem()
}
} else if kind != reflect.Struct {
scope.Err(errors.New("unsupported destination, should be slice or struct"))
return
}
scope.prepareQuerySQL()
if !scope.HasError() {
scope.db.RowsAffected = 0
if str, ok := scope.Get("gorm:query_option"); ok {
scope.SQL += addExtraSpaceIfExist(fmt.Sprint(str))
}
if rows, err := scope.SQLDB().Query(scope.SQL, scope.SQLVars...); scope.Err(err) == nil {
defer rows.Close()
columns, _ := rows.Columns()
for rows.Next() {
scope.db.RowsAffected++
elem := results
if isSlice {
elem = reflect.New(resultType).Elem()
}
scope.scan(rows, columns, scope.New(elem.Addr().Interface()).Fields())
if isSlice {
if isPtr {
results.Set(reflect.Append(results, elem.Addr()))
} else {
results.Set(reflect.Append(results, elem))
}
}
}
if err := rows.Err(); err != nil {
scope.Err(err)
} else if scope.db.RowsAffected == 0 && !isSlice {
scope.Err(ErrRecordNotFound)
}
}
}
}核心的步骤在于 scope.prepareQuerySQL() 构建 SQL 语句.
然后通过 rows, err := scope.SQLDB().Query(scope.SQL, scope.SQLVars...), 执行了数据库查询.
那么查询到的结果是如何传递的, 传递给谁呢?
函数的开头定义了 results = scope.IndirectValue(), 这就是最终查询结果的归属地.
results 只能是结构体或者是结构体的切片.
if kind := results.Kind(); kind == reflect.Slice {
isSlice = true
resultType = results.Type().Elem()
results.Set(reflect.MakeSlice(results.Type(), 0, 0))
if resultType.Kind() == reflect.Ptr {
isPtr = true
resultType = resultType.Elem()
}
} else if kind != reflect.Struct {
scope.Err(errors.New("unsupported destination, should be slice or struct"))
return
}具体如何处理查询到的结果是在下面这部分代码中:
columns, _ := rows.Columns()
for rows.Next() {
scope.db.RowsAffected++
elem := results
if isSlice {
elem = reflect.New(resultType).Elem()
}
scope.scan(rows, columns, scope.New(elem.Addr().Interface()).Fields())
if isSlice {
if isPtr {
results.Set(reflect.Append(results, elem.Addr()))
} else {
results.Set(reflect.Append(results, elem))
}
}
}这部分代码的核心语句在于 scope.scan, 看一下这个方法的定义:
func (scope *Scope) scan(rows *sql.Rows, columns []string, fields []*Field) {
var (
ignored interface{}
values = make([]interface{}, len(columns))
selectFields []*Field
selectedColumnsMap = map[string]int{}
resetFields = map[int]*Field{}
)
for index, column := range columns {
values[index] = &ignored
selectFields = fields
offset := 0
if idx, ok := selectedColumnsMap[column]; ok {
offset = idx + 1
selectFields = selectFields[offset:]
}
for fieldIndex, field := range selectFields {
if field.DBName == column {
if field.Field.Kind() == reflect.Ptr {
values[index] = field.Field.Addr().Interface()
} else {
reflectValue := reflect.New(reflect.PtrTo(field.Struct.Type))
reflectValue.Elem().Set(field.Field.Addr())
values[index] = reflectValue.Interface()
resetFields[index] = field
}
selectedColumnsMap[column] = offset + fieldIndex
if field.IsNormal {
break
}
}
}
}
scope.Err(rows.Scan(values...))
for index, field := range resetFields {
if v := reflect.ValueOf(values[index]).Elem().Elem(); v.IsValid() {
field.Field.Set(v)
}
}
}就和它的名字暗示的那样, 实际上就是调用了 rows.Scan(values...), 将查询到的数据复制到对应的字段中.
由此, 我们就了解了查询时的主要流程了.
前面专注于流程, 略过了构建 SQL 语句的细节, 来仔细看看 prepareQuerySQL 方法.
构建查询 SQL 语句
func (scope *Scope) prepareQuerySQL() {
if scope.Search.raw {
scope.Raw(scope.CombinedConditionSql())
} else {
scope.Raw(fmt.Sprintf("SELECT %v FROM %v %v", scope.selectSQL(), scope.QuotedTableName(), scope.CombinedConditionSql()))
}
return
}内部分支中都使用到了 scope.Raw, 看一下它的实现:
// Raw set raw sql
func (scope *Scope) Raw(sql string) *Scope {
scope.SQL = strings.Replace(sql, "$$$", "?", -1)
return scope
}它的作用是将获取到的 sql 语句赋值到 scope.SQL 字段上, 其中替换了所有的 $$$ 为 ?.
回到 prepareQuerySQL 上来, 重要的部分是其实是 Raw 的参数.
if 的后半部分更好理解点, 就是构建了 SELECT 表达式.
SELECT 表达式需要三个变量, 字段名, 表名, 条件.
将每个都看一下吧.
func (scope *Scope) selectSQL() string {
if len(scope.Search.selects) == 0 {
if len(scope.Search.joinConditions) > 0 {
return fmt.Sprintf("%v.*", scope.QuotedTableName())
}
return "*"
}
return scope.buildSelectQuery(scope.Search.selects)
}
func (scope *Scope) buildSelectQuery(clause map[string]interface{}) (str string) {
switch value := clause["query"].(type) {
case string:
str = value
case []string:
str = strings.Join(value, ", ")
}
args := clause["args"].([]interface{})
replacements := []string{}
for _, arg := range args {
switch reflect.ValueOf(arg).Kind() {
case reflect.Slice:
values := reflect.ValueOf(arg)
var tempMarks []string
for i := 0; i < values.Len(); i++ {
tempMarks = append(tempMarks, scope.AddToVars(values.Index(i).Interface()))
}
replacements = append(replacements, strings.Join(tempMarks, ","))
default:
if valuer, ok := interface{}(arg).(driver.Valuer); ok {
arg, _ = valuer.Value()
}
replacements = append(replacements, scope.AddToVars(arg))
}
}
buff := bytes.NewBuffer([]byte{})
i := 0
for pos, char := range str {
if str[pos] == '?' {
buff.WriteString(replacements[i])
i++
} else {
buff.WriteRune(char)
}
}
str = buff.String()
return
}当 scope.Search.selects 为空的时候, 比较简单.
只要根据是否有连表查询, 返回 table.* 或 *.
buildSelectQuery 就是根据 scope.Search.selects 构建查询字段名.
前面半部分一看就明白.
switch value := clause["query"].(type) {
case string:
str = value
case []string:
str = strings.Join(value, ", ")
}重点是遇到参数时如何处理, 也就是后半段代码.
args := clause["args"].([]interface{})
replacements := []string{}
for _, arg := range args {
switch reflect.ValueOf(arg).Kind() {
case reflect.Slice:
values := reflect.ValueOf(arg)
var tempMarks []string
for i := 0; i < values.Len(); i++ {
tempMarks = append(tempMarks, scope.AddToVars(values.Index(i).Interface()))
}
replacements = append(replacements, strings.Join(tempMarks, ","))
default:
if valuer, ok := interface{}(arg).(driver.Valuer); ok {
arg, _ = valuer.Value()
}
replacements = append(replacements, scope.AddToVars(arg))
}
}
buff := bytes.NewBuffer([]byte{})
i := 0
for pos, char := range str {
if str[pos] == '?' {
buff.WriteString(replacements[i])
i++
} else {
buff.WriteRune(char)
}
}主要的过程是遍历 args := clause["args"].([]interface{}),
创建了一个 replacements 切片. 然后将 str 中所有的 ?,
替换为了对应的字段.
到此, 构建 SELECT 字段的过程就结束了.
获取表名的过程相对简单, 直接展示代码吧:
// QuotedTableName return quoted table name
func (scope *Scope) QuotedTableName() (name string) {
if scope.search != nil && len(scope.Search.tableName) > 0 {
if strings.Contains(scope.Search.tableName, " ") {
return scope.Search.tableName
}
return scope.Quote(scope.Search.tableName)
}
return scope.Quote(scope.TableName())
}条件语句
更多的关注点在于如何构建筛选条件, 即 CombinedConditionSql 方法.
// CombinedConditionSql return combined condition sql
func (scope *Scope) CombinedConditionSql() string {
joinSQL := scope.joinsSQL()
whereSQL := scope.whereSQL()
if scope.Search.raw {
whereSQL = strings.TrimSuffix(strings.TrimPrefix(whereSQL, "WHERE ("), ")")
}
return joinSQL + whereSQL + scope.groupSQL() +
scope.havingSQL() + scope.orderSQL() + scope.limitAndOffsetSQL()
}短小的代码中是精简的逻辑, 条件语句有很多模块, 这里总共有 6 个子句.
都看一遍吧, 看完之后应该对如何构建条件语句不会陌生了.
func (scope *Scope) joinsSQL() string {
var joinConditions []string
for _, clause := range scope.Search.joinConditions {
if sql := scope.buildCondition(clause, true); sql != "" {
joinConditions = append(joinConditions, strings.TrimSuffix(strings.TrimPrefix(sql, "("), ")"))
}
}
return strings.Join(joinConditions, " ") + " "
}创建 joinSQL 的过程中主要用到了 buildCondition, 继续深入:
func (scope *Scope) buildCondition(clause map[string]interface{}, include bool) (str string) {
var (
quotedTableName = scope.QuotedTableName()
quotedPrimaryKey = scope.Quote(scope.PrimaryKey())
equalSQL = "="
inSQL = "IN"
)
// If building not conditions
if !include {
equalSQL = "<>"
inSQL = "NOT IN"
}
switch value := clause["query"].(type) {
case sql.NullInt64:
return fmt.Sprintf("(%v.%v %s %v)", quotedTableName, quotedPrimaryKey, equalSQL, value.Int64)
case int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64:
return fmt.Sprintf("(%v.%v %s %v)", quotedTableName, quotedPrimaryKey, equalSQL, value)
case []int, []int8, []int16, []int32, []int64, []uint, []uint8, []uint16, []uint32, []uint64, []string, []interface{}:
if !include && reflect.ValueOf(value).Len() == 0 {
return
}
str = fmt.Sprintf("(%v.%v %s (?))", quotedTableName, quotedPrimaryKey, inSQL)
clause["args"] = []interface{}{value}
case string:
if isNumberRegexp.MatchString(value) {
return fmt.Sprintf("(%v.%v %s %v)", quotedTableName, quotedPrimaryKey, equalSQL, scope.AddToVars(value))
}
if value != "" {
if !include {
if comparisonRegexp.MatchString(value) {
str = fmt.Sprintf("NOT (%v)", value)
} else {
str = fmt.Sprintf("(%v.%v NOT IN (?))", quotedTableName, scope.Quote(value))
}
} else {
str = fmt.Sprintf("(%v)", value)
}
}
case map[string]interface{}:
var sqls []string
for key, value := range value {
if value != nil {
sqls = append(sqls, fmt.Sprintf("(%v.%v %s %v)", quotedTableName, scope.Quote(key), equalSQL, scope.AddToVars(value)))
} else {
if !include {
sqls = append(sqls, fmt.Sprintf("(%v.%v IS NOT NULL)", quotedTableName, scope.Quote(key)))
} else {
sqls = append(sqls, fmt.Sprintf("(%v.%v IS NULL)", quotedTableName, scope.Quote(key)))
}
}
}
return strings.Join(sqls, " AND ")
case interface{}:
var sqls []string
newScope := scope.New(value)
if len(newScope.Fields()) == 0 {
scope.Err(fmt.Errorf("invalid query condition: %v", value))
return
}
scopeQuotedTableName := newScope.QuotedTableName()
for _, field := range newScope.Fields() {
if !field.IsIgnored && !field.IsBlank {
sqls = append(sqls, fmt.Sprintf("(%v.%v %s %v)", scopeQuotedTableName, scope.Quote(field.DBName), equalSQL, scope.AddToVars(field.Field.Interface())))
}
}
return strings.Join(sqls, " AND ")
default:
scope.Err(fmt.Errorf("invalid query condition: %v", value))
return
}
replacements := []string{}
args := clause["args"].([]interface{})
for _, arg := range args {
var err error
switch reflect.ValueOf(arg).Kind() {
case reflect.Slice: // For where("id in (?)", []int64{1,2})
if scanner, ok := interface{}(arg).(driver.Valuer); ok {
arg, err = scanner.Value()
replacements = append(replacements, scope.AddToVars(arg))
} else if b, ok := arg.([]byte); ok {
replacements = append(replacements, scope.AddToVars(b))
} else if as, ok := arg.([][]interface{}); ok {
var tempMarks []string
for _, a := range as {
var arrayMarks []string
for _, v := range a {
arrayMarks = append(arrayMarks, scope.AddToVars(v))
}
if len(arrayMarks) > 0 {
tempMarks = append(tempMarks, fmt.Sprintf("(%v)", strings.Join(arrayMarks, ",")))
}
}
if len(tempMarks) > 0 {
replacements = append(replacements, strings.Join(tempMarks, ","))
}
} else if values := reflect.ValueOf(arg); values.Len() > 0 {
var tempMarks []string
for i := 0; i < values.Len(); i++ {
tempMarks = append(tempMarks, scope.AddToVars(values.Index(i).Interface()))
}
replacements = append(replacements, strings.Join(tempMarks, ","))
} else {
replacements = append(replacements, scope.AddToVars(Expr("NULL")))
}
default:
if valuer, ok := interface{}(arg).(driver.Valuer); ok {
arg, err = valuer.Value()
}
replacements = append(replacements, scope.AddToVars(arg))
}
if err != nil {
scope.Err(err)
}
}
buff := bytes.NewBuffer([]byte{})
i := 0
for _, s := range str {
if s == '?' && len(replacements) > i {
buff.WriteString(replacements[i])
i++
} else {
buff.WriteRune(s)
}
}
str = buff.String()
return
}开头是一个精妙的选择, 基于 include, 实现了 not 条件.
var (
quotedTableName = scope.QuotedTableName()
quotedPrimaryKey = scope.Quote(scope.PrimaryKey())
equalSQL = "="
inSQL = "IN"
)
// If building not conditions
if !include {
equalSQL = "<>"
inSQL = "NOT IN"
}中间是一个 switch value := clause["query"].(type) 选择.
在这个 switch 选择中, 大部分的条件都会直接返回.
剩余的部分, 则会构建 str 字符串变量.
而这会继续进入到结尾部分, 这部分的代码和我们上面看过的非常类似,
就是根据 clause["args"] 构建 replacements 切片,
用来替换 str 变量中的 ?.
接着看下一个 whereSQL 方法.
func (scope *Scope) whereSQL() (sql string) {
var (
quotedTableName = scope.QuotedTableName()
deletedAtField, hasDeletedAtField = scope.FieldByName("DeletedAt")
primaryConditions, andConditions, orConditions []string
)
if !scope.Search.Unscoped && hasDeletedAtField {
sql := fmt.Sprintf("%v.%v IS NULL", quotedTableName, scope.Quote(deletedAtField.DBName))
primaryConditions = append(primaryConditions, sql)
}
if !scope.PrimaryKeyZero() {
for _, field := range scope.PrimaryFields() {
sql := fmt.Sprintf("%v.%v = %v", quotedTableName, scope.Quote(field.DBName), scope.AddToVars(field.Field.Interface()))
primaryConditions = append(primaryConditions, sql)
}
}
for _, clause := range scope.Search.whereConditions {
if sql := scope.buildCondition(clause, true); sql != "" {
andConditions = append(andConditions, sql)
}
}
for _, clause := range scope.Search.orConditions {
if sql := scope.buildCondition(clause, true); sql != "" {
orConditions = append(orConditions, sql)
}
}
for _, clause := range scope.Search.notConditions {
if sql := scope.buildCondition(clause, false); sql != "" {
andConditions = append(andConditions, sql)
}
}
orSQL := strings.Join(orConditions, " OR ")
combinedSQL := strings.Join(andConditions, " AND ")
if len(combinedSQL) > 0 {
if len(orSQL) > 0 {
combinedSQL = combinedSQL + " OR " + orSQL
}
} else {
combinedSQL = orSQL
}
if len(primaryConditions) > 0 {
sql = "WHERE " + strings.Join(primaryConditions, " AND ")
if len(combinedSQL) > 0 {
sql = sql + " AND (" + combinedSQL + ")"
}
} else if len(combinedSQL) > 0 {
sql = "WHERE " + combinedSQL
}
return
}主要构建了三个部分, primaryConditions, andConditions, orConditions.
if !scope.Search.Unscoped && hasDeletedAtField {
sql := fmt.Sprintf("%v.%v IS NULL", quotedTableName, scope.Quote(deletedAtField.DBName))
primaryConditions = append(primaryConditions, sql)
}
if !scope.PrimaryKeyZero() {
for _, field := range scope.PrimaryFields() {
sql := fmt.Sprintf("%v.%v = %v", quotedTableName, scope.Quote(field.DBName), scope.AddToVars(field.Field.Interface()))
primaryConditions = append(primaryConditions, sql)
}
}前面两个 if 构建了 primaryConditions 条件.
for _, clause := range scope.Search.whereConditions {
if sql := scope.buildCondition(clause, true); sql != "" {
andConditions = append(andConditions, sql)
}
}
for _, clause := range scope.Search.orConditions {
if sql := scope.buildCondition(clause, true); sql != "" {
orConditions = append(orConditions, sql)
}
}
for _, clause := range scope.Search.notConditions {
if sql := scope.buildCondition(clause, false); sql != "" {
andConditions = append(andConditions, sql)
}
}然后三个 for 循环都使用了 buildCondition 方法.
注意到 scope.Search.notConditions 是算在 andConditions 中的.
orSQL := strings.Join(orConditions, " OR ")
combinedSQL := strings.Join(andConditions, " AND ")
if len(combinedSQL) > 0 {
if len(orSQL) > 0 {
combinedSQL = combinedSQL + " OR " + orSQL
}
} else {
combinedSQL = orSQL
}结合 orConditions 和 andConditions 生成了条件语句.
if len(primaryConditions) > 0 {
sql = "WHERE " + strings.Join(primaryConditions, " AND ")
if len(combinedSQL) > 0 {
sql = sql + " AND (" + combinedSQL + ")"
}
} else if len(combinedSQL) > 0 {
sql = "WHERE " + combinedSQL
}
return最后, 结合 primaryConditions 生成最终的 WHERE 子句.
接着看另一个:
func (scope *Scope) groupSQL() string {
if len(scope.Search.group) == 0 {
return ""
}
return " GROUP BY " + scope.Search.group
}GROUP BY 子句比较简单, 直接就能构建.
继续:
func (scope *Scope) havingSQL() string {
if len(scope.Search.havingConditions) == 0 {
return ""
}
var andConditions []string
for _, clause := range scope.Search.havingConditions {
if sql := scope.buildCondition(clause, true); sql != "" {
andConditions = append(andConditions, sql)
}
}
combinedSQL := strings.Join(andConditions, " AND ")
if len(combinedSQL) == 0 {
return ""
}
return " HAVING " + combinedSQL
}HAVING 子句也不算难, 构建完条件之后用 AND 连接, 然后在最前面加上 HAVING 就行了.
继续:
func (scope *Scope) orderSQL() string {
if len(scope.Search.orders) == 0 || scope.Search.ignoreOrderQuery {
return ""
}
var orders []string
for _, order := range scope.Search.orders {
if str, ok := order.(string); ok {
orders = append(orders, scope.quoteIfPossible(str))
} else if expr, ok := order.(*expr); ok {
exp := expr.expr
for _, arg := range expr.args {
exp = strings.Replace(exp, "?", scope.AddToVars(arg), 1)
}
orders = append(orders, exp)
}
}
return " ORDER BY " + strings.Join(orders, ",")
}结构也是类似, 遍历 scope.Search.orders 切片, order 有两种不同的类型, 字符串或者 expr 结构体.
后者用于处理带参数的情况.
最后还有一个 limitAndOffsetSQL 方法:
func (scope *Scope) limitAndOffsetSQL() string {
return scope.Dialect().LimitAndOffsetSQL(scope.Search.limit, scope.Search.offset)
}这直接调用了具体数据库驱动中的 LimitAndOffsetSQL 方法.
看两个具体的实现, 一个是通用中的实现, 另一个是 mysql 中的实现.
func (commonDialect) LimitAndOffsetSQL(limit, offset interface{}) (sql string) {
if limit != nil {
if parsedLimit, err := strconv.ParseInt(fmt.Sprint(limit), 0, 0); err == nil && parsedLimit >= 0 {
sql += fmt.Sprintf(" LIMIT %d", parsedLimit)
}
}
if offset != nil {
if parsedOffset, err := strconv.ParseInt(fmt.Sprint(offset), 0, 0); err == nil && parsedOffset >= 0 {
sql += fmt.Sprintf(" OFFSET %d", parsedOffset)
}
}
return
}直接将 limit 和 offset 解析为 int 类型, 然后连接对应的关键字即可.
接着看一下 mysql 中的实现:
func (s mysql) LimitAndOffsetSQL(limit, offset interface{}) (sql string) {
if limit != nil {
if parsedLimit, err := strconv.ParseInt(fmt.Sprint(limit), 0, 0); err == nil && parsedLimit >= 0 {
sql += fmt.Sprintf(" LIMIT %d", parsedLimit)
if offset != nil {
if parsedOffset, err := strconv.ParseInt(fmt.Sprint(offset), 0, 0); err == nil && parsedOffset >= 0 {
sql += fmt.Sprintf(" OFFSET %d", parsedOffset)
}
}
}
}
return
}两者的区别在于 offset 的嵌套, mysql 中 offset 必须和 limit 一起使用.
就这样, CombinedConditionSql 中的所有子句都看完了.
说到底其实也没什么魔法, 不过是根据不同的条件, 构建不同的 SQL 语句.
小结
一路从 First 深入到查询的内部细节. 在了解了底层细节之后, 其他类似的方法也就不难理解了.
// Take return a record that match given conditions, the order will depend on the database implementation
func (s *DB) Take(out interface{}, where ...interface{}) *DB {
newScope := s.NewScope(out)
newScope.Search.Limit(1)
return newScope.inlineCondition(where...).callCallbacks(s.parent.callbacks.queries).db
}
// Last find last record that match given conditions, order by primary key
func (s *DB) Last(out interface{}, where ...interface{}) *DB {
newScope := s.NewScope(out)
newScope.Search.Limit(1)
return newScope.Set("gorm:order_by_primary_key", "DESC").
inlineCondition(where...).callCallbacks(s.parent.callbacks.queries).db
}
// Find find records that match given conditions
func (s *DB) Find(out interface{}, where ...interface{}) *DB {
return s.NewScope(out).inlineCondition(where...).callCallbacks(s.parent.callbacks.queries).db
}search 结构体
前面的过程中, 我们只看到了最简单的查询是如何产生的.
在这个过程中, 没有仔细研究查询条件是如何存储的.
看一下如何使用 Where 方法添加查询条件.
// Get first matched record
db.Where("name = ?", "jinzhu").First(&user)
//// SELECT * FROM users WHERE name = 'jinzhu' limit 1;
// Get all matched records
db.Where("name = ?", "jinzhu").Find(&users)
//// SELECT * FROM users WHERE name = 'jinzhu';上面的例子来自于官方文档. GORM 使用链式调用的风格, 可以串联多个 Where 方法, 或是其他的查询条件.
// Where return a new relation, filter records with given conditions, accepts `map`, `struct` or `string` as conditions, refer http://jinzhu.github.io/gorm/crud.html#query
func (s *DB) Where(query interface{}, args ...interface{}) *DB {
return s.clone().search.Where(query, args...).db
}上面是 Where 方法的代码, 在它的源码附近有很多类似的的方法.
// Or filter records that match before conditions or this one, similar to `Where`
func (s *DB) Or(query interface{}, args ...interface{}) *DB {
return s.clone().search.Or(query, args...).db
}
// Not filter records that don't match current conditions, similar to `Where`
func (s *DB) Not(query interface{}, args ...interface{}) *DB {
return s.clone().search.Not(query, args...).db
}可以很容易的发现, 这一切的源头都是 search 对象.
结构体 DB 定义的时候, 有个字段就是 search:
search *searchsearch 的定义
这就是用于存储查询条件的地方. 它的定义如下:
type search struct {
db *DB
whereConditions []map[string]interface{}
orConditions []map[string]interface{}
notConditions []map[string]interface{}
havingConditions []map[string]interface{}
joinConditions []map[string]interface{}
initAttrs []interface{}
assignAttrs []interface{}
selects map[string]interface{}
omits []string
orders []interface{}
preload []searchPreload
offset interface{}
limit interface{}
group string
tableName string
raw bool
Unscoped bool
ignoreOrderQuery bool
}
type searchPreload struct {
schema string
conditions []interface{}
}这里有很多类型为 []map[string]interface{} 的字段, 结合前面关于条件查询的代码, 就能回忆起这就是存储各种条件的地方.
另一些字段比如 offset 和 limit 也很容易明白它的作用.
search 的方法
search 下有很多方法, 虽然方法数量比较多, 但基本都很短, 总共也就一百行出头.
func (s *search) clone() *search {
clone := *s
return &clone
}这个克隆方法有点独特, 似乎什么也没做, 也可能是我见识少.
func (s *search) Where(query interface{}, values ...interface{}) *search {
s.whereConditions = append(s.whereConditions, map[string]interface{}{"query": query, "args": values})
return s
}
func (s *search) Not(query interface{}, values ...interface{}) *search {
s.notConditions = append(s.notConditions, map[string]interface{}{"query": query, "args": values})
return s
}
func (s *search) Or(query interface{}, values ...interface{}) *search {
s.orConditions = append(s.orConditions, map[string]interface{}{"query": query, "args": values})
return s
}上面这些方法都是用参数构建成一个 map 然后推入对应的切片中, 考虑到链式调用, 返回了本身.
func (s *search) Attrs(attrs ...interface{}) *search {
s.initAttrs = append(s.initAttrs, toSearchableMap(attrs...))
return s
}
func (s *search) Assign(attrs ...interface{}) *search {
s.assignAttrs = append(s.assignAttrs, toSearchableMap(attrs...))
return s
}
func toSearchableMap(attrs ...interface{}) (result interface{}) {
if len(attrs) > 1 {
if str, ok := attrs[0].(string); ok {
result = map[string]interface{}{str: attrs[1]}
}
} else if len(attrs) == 1 {
if attr, ok := attrs[0].(map[string]interface{}); ok {
result = attr
}
if attr, ok := attrs[0].(interface{}); ok {
result = attr
}
}
return
}这两个方法也是类似, 并使用了 toSearchableMap 转换参数.
func (s *search) Order(value interface{}, reorder ...bool) *search {
if len(reorder) > 0 && reorder[0] {
s.orders = []interface{}{}
}
if value != nil && value != "" {
s.orders = append(s.orders, value)
}
return s
}看到这个可能有点疑惑, 可以从文档和注释中获取解释.
// Order specify order when retrieve records from database, set reorder to `true` to overwrite defined conditions
// db.Order("name DESC")
// db.Order("name DESC", true) // reorder
// db.Order(gorm.Expr("name = ? DESC", "first")) // sql expression
func (s *DB) Order(value interface{}, reorder ...bool) *DB {
return s.clone().search.Order(value, reorder...).db
}第二个参数用于判断是否覆盖前面的排序条件.
可能有点奇怪的是为什么 reorder 是可变参数, 不知为了兼容或者是历史遗留.
另一点是不能理解 []interface{}{}, 这其实可以分为两部分, []interface{} 是类型, {} 构造了一个空的该类型实例.
func (s *search) Select(query interface{}, args ...interface{}) *search {
s.selects = map[string]interface{}{"query": query, "args": args}
return s
}
func (s *search) Omit(columns ...string) *search {
s.omits = columns
return s
}
func (s *search) Limit(limit interface{}) *search {
s.limit = limit
return s
}
func (s *search) Offset(offset interface{}) *search {
s.offset = offset
return s
}这几个就是替换型的了, 每次调用都只会保存最新值.
func (s *search) Group(query string) *search {
s.group = s.getInterfaceAsSQL(query)
return s
}
func (s *search) getInterfaceAsSQL(value interface{}) (str string) {
switch value.(type) {
case string, int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64:
str = fmt.Sprintf("%v", value)
default:
s.db.AddError(ErrInvalidSQL)
}
if str == "-1" {
return ""
}
return
}getInterfaceAsSQL 的一个特性是使用 -1 会重置.
func (s *search) Having(query interface{}, values ...interface{}) *search {
if val, ok := query.(*expr); ok {
s.havingConditions = append(s.havingConditions, map[string]interface{}{"query": val.expr, "args": val.args})
} else {
s.havingConditions = append(s.havingConditions, map[string]interface{}{"query": query, "args": values})
}
return s
}
func (s *search) Joins(query string, values ...interface{}) *search {
s.joinConditions = append(s.joinConditions, map[string]interface{}{"query": query, "args": values})
return s
}这其实也比较类似前面看过的, 就不多解释了.
func (s *search) Preload(schema string, values ...interface{}) *search {
var preloads []searchPreload
for _, preload := range s.preload {
if preload.schema != schema {
preloads = append(preloads, preload)
}
}
preloads = append(preloads, searchPreload{schema, values})
s.preload = preloads
return s
}Preload 需要防止重复, 所以开头会重新遍历一遍已经存在的 schema.
func (s *search) Raw(b bool) *search {
s.raw = b
return s
}
func (s *search) unscoped() *search {
s.Unscoped = true
return s
}
func (s *search) Table(name string) *search {
s.tableName = name
return s
}最后几个方法也没什么特殊的.
小结
search 结构体还是挺简单的, 定义加方法总共也就一百多行.
但用处却不小, 查询相关的条件都是存储在这里的.
总结
这部分主要查看了 SQL 查询是如何发生的, 并在这个过程中探索了各种查询子句是如何实现的. 同时, 也研究了一下 search 结构体和它的作用.
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