源码分析：GORM是如何生成sql的

在gorm下实现sqlcommenter过程中，遇到一些问题，顺便把gorm整个流程梳理了一遍，整理记录一下。

gorm使用示例

package main

import (
  "gorm.io/driver/mysql"
  "gorm.io/gorm"
)

type Product struct {
  gorm.Model
  Code  string
  Price uint
}

func main() {
  // 参考 https://github.com/go-sql-driver/mysql#dsn-data-source-name 获取详情
  dsn := "user:pass@tcp(127.0.0.1:3306)/dbname?charset=utf8mb4&parseTime=True&loc=Local"
  db, err := gorm.Open(mysql.Open(dsn), &gorm.Config{})
  
  var product Product
  db.First(&product, 1) // 根据整型主键查找
}

我们以First查询为例，看一下是怎么转成具体sql的。

在finisher_api.go文件，声明了First方法。

// First finds the first record ordered by primary key, matching given conditions conds
func (db *DB) First(dest interface{}, conds ...interface{}) (tx *DB) {
	// 注册Order类型的Clause
	tx = db.Limit(1).Order(clause.OrderByColumn{
		Column: clause.Column{Table: clause.CurrentTable, Name: clause.PrimaryKey},
	})
	// 这里如果有指定条件的话，注册一个Where类型的Clause
	if len(conds) > 0 {
		if exprs := tx.Statement.BuildCondition(conds[0], conds[1:]...); len(exprs) > 0 {
			tx.Statement.AddClause(clause.Where{Exprs: exprs})
		}
	}
	tx.Statement.RaiseErrorOnNotFound = true
	tx.Statement.Dest = dest
	return tx.callbacks.Query().Execute(tx)
}

在gorm.go文件，可以找到tx.callbacks定义。

type Config struct {
	...

	callbacks  *callbacks
	cacheStore *sync.Map
}

callbacks.go

// callbacks gorm callbacks manager
type callbacks struct {
	processors map[string]*processor
}

type processor struct {
	db        *DB
	Clauses   []string
	fns       []func(*DB)
	callbacks []*callback
}

type callback struct {
	name      string
	before    string
	after     string
	remove    bool
	replace   bool
	match     func(*DB) bool
	handler   func(*DB)
	processor *processor
}

// 返回query类型的processor
func (cs *callbacks) Query() *processor {
	return cs.processors["query"]
}

func (p *processor) Execute(db *DB) *DB {
	// call scopes
	for len(db.Statement.scopes) > 0 {
		db = db.executeScopes()
	}

	var (
		curTime           = time.Now()
		stmt              = db.Statement
		resetBuildClauses bool
	)

	// 注意这里的stmt.BuildClauses，后面会用到这个信息
	if len(stmt.BuildClauses) == 0 {
		stmt.BuildClauses = p.Clauses
		resetBuildClauses = true
	}

	if optimizer, ok := db.Statement.Dest.(StatementModifier); ok {
		optimizer.ModifyStatement(stmt)
	}

	// assign model values
	if stmt.Model == nil {
		stmt.Model = stmt.Dest
	} else if stmt.Dest == nil {
		stmt.Dest = stmt.Model
	}

	// parse model values
	if stmt.Model != nil {
		if err := stmt.Parse(stmt.Model); err != nil && (!errors.Is(err, schema.ErrUnsupportedDataType) || (stmt.Table == "" && stmt.TableExpr == nil && stmt.SQL.Len() == 0)) {
			if errors.Is(err, schema.ErrUnsupportedDataType) && stmt.Table == "" && stmt.TableExpr == nil {
				db.AddError(fmt.Errorf("%w: Table not set, please set it like: db.Model(&user) or db.Table(\"users\")", err))
			} else {
				db.AddError(err)
			}
		}
	}

	// assign stmt.ReflectValue
	if stmt.Dest != nil {
		stmt.ReflectValue = reflect.ValueOf(stmt.Dest)
		for stmt.ReflectValue.Kind() == reflect.Ptr {
			if stmt.ReflectValue.IsNil() && stmt.ReflectValue.CanAddr() {
				stmt.ReflectValue.Set(reflect.New(stmt.ReflectValue.Type().Elem()))
			}

			stmt.ReflectValue = stmt.ReflectValue.Elem()
		}
		if !stmt.ReflectValue.IsValid() {
			db.AddError(ErrInvalidValue)
		}
	}

	// 根据优先级执行不同callback的回调方法
	for _, f := range p.fns {
		f(db)
	}

	if stmt.SQL.Len() > 0 {
		db.Logger.Trace(stmt.Context, curTime, func() (string, int64) {
			sql, vars := stmt.SQL.String(), stmt.Vars
			if filter, ok := db.Logger.(ParamsFilter); ok {
				sql, vars = filter.ParamsFilter(stmt.Context, stmt.SQL.String(), stmt.Vars...)
			}
			return db.Dialector.Explain(sql, vars...), db.RowsAffected
		}, db.Error)
	}

	if !stmt.DB.DryRun {
		stmt.SQL.Reset()
		stmt.Vars = nil
	}

	if resetBuildClauses {
		stmt.BuildClauses = nil
	}

	return db
}

接下来，我们来看一下内置的callback是如何注册的。

mysql.go

var (
	// CreateClauses create clauses
	CreateClauses = []string{"INSERT", "VALUES", "ON CONFLICT"}
	// QueryClauses query clauses
	QueryClauses = []string{}
	// UpdateClauses update clauses
	UpdateClauses = []string{"UPDATE", "SET", "WHERE", "ORDER BY", "LIMIT"}
	// DeleteClauses delete clauses
	DeleteClauses = []string{"DELETE", "FROM", "WHERE", "ORDER BY", "LIMIT"}

	defaultDatetimePrecision = 3
)

...

func (dialector Dialector) Initialize(db *gorm.DB) (err error) {
	if dialector.DriverName == "" {
		dialector.DriverName = "mysql"
	}

	if dialector.DefaultDatetimePrecision == nil {
		dialector.DefaultDatetimePrecision = &defaultDatetimePrecision
	}

	if dialector.Conn != nil {
		db.ConnPool = dialector.Conn
	} else {
		db.ConnPool, err = sql.Open(dialector.DriverName, dialector.DSN)
		if err != nil {
			return err
		}
	}

	withReturning := false
	if !dialector.Config.SkipInitializeWithVersion {
		err = db.ConnPool.QueryRowContext(context.Background(), "SELECT VERSION()").Scan(&dialector.ServerVersion)
		if err != nil {
			return err
		}

		if strings.Contains(dialector.ServerVersion, "MariaDB") {
			dialector.Config.DontSupportRenameIndex = true
			dialector.Config.DontSupportRenameColumn = true
			dialector.Config.DontSupportForShareClause = true
			dialector.Config.DontSupportNullAsDefaultValue = true
			withReturning = checkVersion(dialector.ServerVersion, "10.5")
		} else if strings.HasPrefix(dialector.ServerVersion, "5.6.") {
			dialector.Config.DontSupportRenameIndex = true
			dialector.Config.DontSupportRenameColumn = true
			dialector.Config.DontSupportForShareClause = true
			dialector.Config.DontSupportDropConstraint = true
		} else if strings.HasPrefix(dialector.ServerVersion, "5.7.") {
			dialector.Config.DontSupportRenameColumn = true
			dialector.Config.DontSupportForShareClause = true
			dialector.Config.DontSupportDropConstraint = true
		} else if strings.HasPrefix(dialector.ServerVersion, "5.") {
			dialector.Config.DisableDatetimePrecision = true
			dialector.Config.DontSupportRenameIndex = true
			dialector.Config.DontSupportRenameColumn = true
			dialector.Config.DontSupportForShareClause = true
			dialector.Config.DontSupportDropConstraint = true
		}

		if strings.Contains(dialector.ServerVersion, "TiDB") {
			dialector.Config.DontSupportRenameColumnUnique = true
		}
	}

	// register callbacks
	callbackConfig := &callbacks.Config{
		CreateClauses: CreateClauses,
		QueryClauses:  QueryClauses,
		UpdateClauses: UpdateClauses,
		DeleteClauses: DeleteClauses,
	}

	if !dialector.Config.DisableWithReturning && withReturning {
		if !utils.Contains(callbackConfig.CreateClauses, "RETURNING") {
			callbackConfig.CreateClauses = append(callbackConfig.CreateClauses, "RETURNING")
		}

		if !utils.Contains(callbackConfig.UpdateClauses, "RETURNING") {
			callbackConfig.UpdateClauses = append(callbackConfig.UpdateClauses, "RETURNING")
		}

		if !utils.Contains(callbackConfig.DeleteClauses, "RETURNING") {
			callbackConfig.DeleteClauses = append(callbackConfig.DeleteClauses, "RETURNING")
		}
	}

	// 注册默认callback
	callbacks.RegisterDefaultCallbacks(db, callbackConfig)

	for k, v := range dialector.ClauseBuilders() {
		db.ClauseBuilders[k] = v
	}
	return
}

callbacks.go

var (
	createClauses = []string{"INSERT", "VALUES", "ON CONFLICT"}
	queryClauses  = []string{"SELECT", "FROM", "WHERE", "GROUP BY", "ORDER BY", "LIMIT", "FOR"}
	updateClauses = []string{"UPDATE", "SET", "WHERE"}
	deleteClauses = []string{"DELETE", "FROM", "WHERE"}
)

type Config struct {
	LastInsertIDReversed bool
	CreateClauses        []string
	QueryClauses         []string
	UpdateClauses        []string
	DeleteClauses        []string
}

func RegisterDefaultCallbacks(db *gorm.DB, config *Config) {
	enableTransaction := func(db *gorm.DB) bool {
		return !db.SkipDefaultTransaction
	}

	if len(config.CreateClauses) == 0 {
		config.CreateClauses = createClauses
	}
	if len(config.QueryClauses) == 0 {
		config.QueryClauses = queryClauses
	}
	if len(config.DeleteClauses) == 0 {
		config.DeleteClauses = deleteClauses
	}
	if len(config.UpdateClauses) == 0 {
		config.UpdateClauses = updateClauses
	}

    // 注册不同类型的callback
	createCallback := db.Callback().Create()
	createCallback.Match(enableTransaction).Register("gorm:begin_transaction", BeginTransaction)
	createCallback.Register("gorm:before_create", BeforeCreate)
	createCallback.Register("gorm:save_before_associations", SaveBeforeAssociations(true))
	createCallback.Register("gorm:create", Create(config))
	createCallback.Register("gorm:save_after_associations", SaveAfterAssociations(true))
	createCallback.Register("gorm:after_create", AfterCreate)
	createCallback.Match(enableTransaction).Register("gorm:commit_or_rollback_transaction", CommitOrRollbackTransaction)
	createCallback.Clauses = config.CreateClauses

	queryCallback := db.Callback().Query()
	queryCallback.Register("gorm:query", Query)
	queryCallback.Register("gorm:preload", Preload)
	queryCallback.Register("gorm:after_query", AfterQuery)
	queryCallback.Clauses = config.QueryClauses

	deleteCallback := db.Callback().Delete()
	deleteCallback.Match(enableTransaction).Register("gorm:begin_transaction", BeginTransaction)
	deleteCallback.Register("gorm:before_delete", BeforeDelete)
	deleteCallback.Register("gorm:delete_before_associations", DeleteBeforeAssociations)
	deleteCallback.Register("gorm:delete", Delete(config))
	deleteCallback.Register("gorm:after_delete", AfterDelete)
	deleteCallback.Match(enableTransaction).Register("gorm:commit_or_rollback_transaction", CommitOrRollbackTransaction)
	deleteCallback.Clauses = config.DeleteClauses

	updateCallback := db.Callback().Update()
	updateCallback.Match(enableTransaction).Register("gorm:begin_transaction", BeginTransaction)
	updateCallback.Register("gorm:setup_reflect_value", SetupUpdateReflectValue)
	updateCallback.Register("gorm:before_update", BeforeUpdate)
	updateCallback.Register("gorm:save_before_associations", SaveBeforeAssociations(false))
	updateCallback.Register("gorm:update", Update(config))
	updateCallback.Register("gorm:save_after_associations", SaveAfterAssociations(false))
	updateCallback.Register("gorm:after_update", AfterUpdate)
	updateCallback.Match(enableTransaction).Register("gorm:commit_or_rollback_transaction", CommitOrRollbackTransaction)
	updateCallback.Clauses = config.UpdateClauses

	rowCallback := db.Callback().Row()
	rowCallback.Register("gorm:row", RowQuery)
	rowCallback.Clauses = config.QueryClauses

	rawCallback := db.Callback().Raw()
	rawCallback.Register("gorm:raw", RawExec)
	rawCallback.Clauses = config.QueryClauses
}

到这里，默认callback就注册完成了，但是是如何转成对应sql的呢？

别急，我们继续往下看。

在RegisterDefaultCallbacks方法里注册了一个gorm:query类型的callback，对应的回调方法为Query。

query.go

func Query(db *gorm.DB) {
	if db.Error == nil {
		// 调用BuildQuerySQL方法
		BuildQuerySQL(db)

		if !db.DryRun && db.Error == nil {
			rows, err := db.Statement.ConnPool.QueryContext(db.Statement.Context, db.Statement.SQL.String(), db.Statement.Vars...)
			if err != nil {
				db.AddError(err)
				return
			}
			defer func() {
				db.AddError(rows.Close())
			}()
			gorm.Scan(rows, db, 0)
		}
	}
}

func BuildQuerySQL(db *gorm.DB) {
	if db.Statement.Schema != nil {
		for _, c := range db.Statement.Schema.QueryClauses {
			db.Statement.AddClause(c)
		}
	}

	if db.Statement.SQL.Len() == 0 {
		db.Statement.SQL.Grow(100)
		clauseSelect := clause.Select{Distinct: db.Statement.Distinct}

		if db.Statement.ReflectValue.Kind() == reflect.Struct && db.Statement.ReflectValue.Type() == db.Statement.Schema.ModelType {
			var conds []clause.Expression
			for _, primaryField := range db.Statement.Schema.PrimaryFields {
				if v, isZero := primaryField.ValueOf(db.Statement.Context, db.Statement.ReflectValue); !isZero {
					conds = append(conds, clause.Eq{Column: clause.Column{Table: db.Statement.Table, Name: primaryField.DBName}, Value: v})
				}
			}

			if len(conds) > 0 {
				db.Statement.AddClause(clause.Where{Exprs: conds})
			}
		}

		if len(db.Statement.Selects) > 0 {
			clauseSelect.Columns = make([]clause.Column, len(db.Statement.Selects))
			for idx, name := range db.Statement.Selects {
				if db.Statement.Schema == nil {
					clauseSelect.Columns[idx] = clause.Column{Name: name, Raw: true}
				} else if f := db.Statement.Schema.LookUpField(name); f != nil {
					clauseSelect.Columns[idx] = clause.Column{Name: f.DBName}
				} else {
					clauseSelect.Columns[idx] = clause.Column{Name: name, Raw: true}
				}
			}
		} else if db.Statement.Schema != nil && len(db.Statement.Omits) > 0 {
			selectColumns, _ := db.Statement.SelectAndOmitColumns(false, false)
			clauseSelect.Columns = make([]clause.Column, 0, len(db.Statement.Schema.DBNames))
			for _, dbName := range db.Statement.Schema.DBNames {
				if v, ok := selectColumns[dbName]; (ok && v) || !ok {
					clauseSelect.Columns = append(clauseSelect.Columns, clause.Column{Table: db.Statement.Table, Name: dbName})
				}
			}
		} else if db.Statement.Schema != nil && db.Statement.ReflectValue.IsValid() {
			queryFields := db.QueryFields
			if !queryFields {
				switch db.Statement.ReflectValue.Kind() {
				case reflect.Struct:
					queryFields = db.Statement.ReflectValue.Type() != db.Statement.Schema.ModelType
				case reflect.Slice:
					queryFields = db.Statement.ReflectValue.Type().Elem() != db.Statement.Schema.ModelType
				}
			}

			if queryFields {
				stmt := gorm.Statement{DB: db}
				// smaller struct
				if err := stmt.Parse(db.Statement.Dest); err == nil && (db.QueryFields || stmt.Schema.ModelType != db.Statement.Schema.ModelType) {
					clauseSelect.Columns = make([]clause.Column, len(stmt.Schema.DBNames))

					for idx, dbName := range stmt.Schema.DBNames {
						clauseSelect.Columns[idx] = clause.Column{Table: db.Statement.Table, Name: dbName}
					}
				}
			}
		}

		// inline joins
		fromClause := clause.From{}
		if v, ok := db.Statement.Clauses["FROM"].Expression.(clause.From); ok {
			fromClause = v
		}

		if len(db.Statement.Joins) != 0 || len(fromClause.Joins) != 0 {
			if len(db.Statement.Selects) == 0 && len(db.Statement.Omits) == 0 && db.Statement.Schema != nil {
				clauseSelect.Columns = make([]clause.Column, len(db.Statement.Schema.DBNames))
				for idx, dbName := range db.Statement.Schema.DBNames {
					clauseSelect.Columns[idx] = clause.Column{Table: db.Statement.Table, Name: dbName}
				}
			}

			specifiedRelationsName := make(map[string]interface{})
			for _, join := range db.Statement.Joins {
				if db.Statement.Schema != nil {
					var isRelations bool // is relations or raw sql
					var relations []*schema.Relationship
					relation, ok := db.Statement.Schema.Relationships.Relations[join.Name]
					if ok {
						isRelations = true
						relations = append(relations, relation)
					} else {
						// handle nested join like "Manager.Company"
						nestedJoinNames := strings.Split(join.Name, ".")
						if len(nestedJoinNames) > 1 {
							isNestedJoin := true
							gussNestedRelations := make([]*schema.Relationship, 0, len(nestedJoinNames))
							currentRelations := db.Statement.Schema.Relationships.Relations
							for _, relname := range nestedJoinNames {
								// incomplete match, only treated as raw sql
								if relation, ok = currentRelations[relname]; ok {
									gussNestedRelations = append(gussNestedRelations, relation)
									currentRelations = relation.FieldSchema.Relationships.Relations
								} else {
									isNestedJoin = false
									break
								}
							}

							if isNestedJoin {
								isRelations = true
								relations = gussNestedRelations
							}
						}
					}

					if isRelations {
						genJoinClause := func(joinType clause.JoinType, parentTableName string, relation *schema.Relationship) clause.Join {
							tableAliasName := relation.Name
							if parentTableName != clause.CurrentTable {
								tableAliasName = utils.NestedRelationName(parentTableName, tableAliasName)
							}

							columnStmt := gorm.Statement{
								Table: tableAliasName, DB: db, Schema: relation.FieldSchema,
								Selects: join.Selects, Omits: join.Omits,
							}

							selectColumns, restricted := columnStmt.SelectAndOmitColumns(false, false)
							for _, s := range relation.FieldSchema.DBNames {
								if v, ok := selectColumns[s]; (ok && v) || (!ok && !restricted) {
									clauseSelect.Columns = append(clauseSelect.Columns, clause.Column{
										Table: tableAliasName,
										Name:  s,
										Alias: utils.NestedRelationName(tableAliasName, s),
									})
								}
							}

							exprs := make([]clause.Expression, len(relation.References))
							for idx, ref := range relation.References {
								if ref.OwnPrimaryKey {
									exprs[idx] = clause.Eq{
										Column: clause.Column{Table: parentTableName, Name: ref.PrimaryKey.DBName},
										Value:  clause.Column{Table: tableAliasName, Name: ref.ForeignKey.DBName},
									}
								} else {
									if ref.PrimaryValue == "" {
										exprs[idx] = clause.Eq{
											Column: clause.Column{Table: parentTableName, Name: ref.ForeignKey.DBName},
											Value:  clause.Column{Table: tableAliasName, Name: ref.PrimaryKey.DBName},
										}
									} else {
										exprs[idx] = clause.Eq{
											Column: clause.Column{Table: tableAliasName, Name: ref.ForeignKey.DBName},
											Value:  ref.PrimaryValue,
										}
									}
								}
							}

							{
								onStmt := gorm.Statement{Table: tableAliasName, DB: db, Clauses: map[string]clause.Clause{}}
								for _, c := range relation.FieldSchema.QueryClauses {
									onStmt.AddClause(c)
								}

								if join.On != nil {
									onStmt.AddClause(join.On)
								}

								if cs, ok := onStmt.Clauses["WHERE"]; ok {
									if where, ok := cs.Expression.(clause.Where); ok {
										where.Build(&onStmt)

										if onSQL := onStmt.SQL.String(); onSQL != "" {
											vars := onStmt.Vars
											for idx, v := range vars {
												bindvar := strings.Builder{}
												onStmt.Vars = vars[0 : idx+1]
												db.Dialector.BindVarTo(&bindvar, &onStmt, v)
												onSQL = strings.Replace(onSQL, bindvar.String(), "?", 1)
											}

											exprs = append(exprs, clause.Expr{SQL: onSQL, Vars: vars})
										}
									}
								}
							}

							return clause.Join{
								Type:  joinType,
								Table: clause.Table{Name: relation.FieldSchema.Table, Alias: tableAliasName},
								ON:    clause.Where{Exprs: exprs},
							}
						}

						parentTableName := clause.CurrentTable
						for _, rel := range relations {
							// joins table alias like "Manager, Company, Manager__Company"
							nestedAlias := utils.NestedRelationName(parentTableName, rel.Name)
							if _, ok := specifiedRelationsName[nestedAlias]; !ok {
								fromClause.Joins = append(fromClause.Joins, genJoinClause(join.JoinType, parentTableName, rel))
								specifiedRelationsName[nestedAlias] = nil
							}

							if parentTableName != clause.CurrentTable {
								parentTableName = utils.NestedRelationName(parentTableName, rel.Name)
							} else {
								parentTableName = rel.Name
							}
						}
					} else {
						fromClause.Joins = append(fromClause.Joins, clause.Join{
							Expression: clause.NamedExpr{SQL: join.Name, Vars: join.Conds},
						})
					}
				} else {
					fromClause.Joins = append(fromClause.Joins, clause.Join{
						Expression: clause.NamedExpr{SQL: join.Name, Vars: join.Conds},
					})
				}
			}

			db.Statement.AddClause(fromClause)
		} else {
			db.Statement.AddClauseIfNotExists(clause.From{})
		}

		db.Statement.AddClauseIfNotExists(clauseSelect)

		// db.Statement.BuildClauses眼熟吗？还记得前面的stmt.BuildClauses吗？
		db.Statement.Build(db.Statement.BuildClauses...)
	}
}

重头戏终于来了，Query方法里调用了BuildQuerySQl，看名字也能猜到这里就是生成sql了，这里最终调用了db.Statement.Build方法。

statement.go

// Build build sql with clauses names
func (stmt *Statement) Build(clauses ...string) {
	var firstClauseWritten bool

	for _, name := range clauses {
		if c, ok := stmt.Clauses[name]; ok {
			if firstClauseWritten {
				stmt.WriteByte(' ')
			}

			firstClauseWritten = true
			if b, ok := stmt.DB.ClauseBuilders[name]; ok {
				b(c, stmt)
			} else {
				c.Build(stmt)
			}
		}
	}
}

这里会根据statement的BuildCluauses属性，执行Clause的Build方法。

clause.go

// ClauseBuilder clause builder, allows to customize how to build clause
type ClauseBuilder func(Clause, Builder)

type Writer interface {
	WriteByte(byte) error
	WriteString(string) (int, error)
}

// Builder builder interface
type Builder interface {
	Writer
	WriteQuoted(field interface{})
	AddVar(Writer, ...interface{})
	AddError(error) error
}

// Clause
type Clause struct {
	Name                string // WHERE
	BeforeExpression    Expression
	AfterNameExpression Expression
	AfterExpression     Expression
	Expression          Expression
	Builder             ClauseBuilder
}

// Build build clause
func (c Clause) Build(builder Builder) {
	if c.Builder != nil {
		c.Builder(c, builder)
	} else if c.Expression != nil {
		if c.BeforeExpression != nil {
			c.BeforeExpression.Build(builder)
			builder.WriteByte(' ')
		}

		if c.Name != "" {
			builder.WriteString(c.Name)
			builder.WriteByte(' ')
		}

		if c.AfterNameExpression != nil {
			c.AfterNameExpression.Build(builder)
			builder.WriteByte(' ')
		}

		c.Expression.Build(builder)

		if c.AfterExpression != nil {
			builder.WriteByte(' ')
			c.AfterExpression.Build(builder)
		}
	}
}

这里会执行对应Clause的Build方法。

// Select select attrs when querying, updating, creating
type Select struct {
	Distinct   bool
	Columns    []Column
	Expression Expression
}

func (s Select) Name() string {
	return "SELECT"
}

func (s Select) Build(builder Builder) {
	if len(s.Columns) > 0 {
		if s.Distinct {
			builder.WriteString("DISTINCT ")
		}

		for idx, column := range s.Columns {
			if idx > 0 {
				builder.WriteByte(',')
			}
			builder.WriteQuoted(column)
		}
	} else {
		builder.WriteByte('*')
	}
}

func (s Select) MergeClause(clause *Clause) {
	if s.Expression != nil {
		if s.Distinct {
			if expr, ok := s.Expression.(Expr); ok {
				expr.SQL = "DISTINCT " + expr.SQL
				clause.Expression = expr
				return
			}
		}

		clause.Expression = s.Expression
	} else {
		clause.Expression = s
	}
}

这是Select类型的Clause定义，是不是一下就清楚了。

gorm通过callback里注册Clause，在Clause里实现了sql拼接操作。

看了几回源码，这次总算是搞清楚了。