link/vendor/gorm.io/gorm/callbacks/preload.go

package callbacks

import (
	"reflect"

	"gorm.io/gorm"
	"gorm.io/gorm/clause"
	"gorm.io/gorm/schema"
	"gorm.io/gorm/utils"
)

func preload(db *gorm.DB, rel *schema.Relationship, conds []interface{}, preloads map[string][]interface{}) {
	var (
		reflectValue     = db.Statement.ReflectValue
		tx               = db.Session(&gorm.Session{NewDB: true}).Model(nil).Session(&gorm.Session{SkipHooks: db.Statement.SkipHooks})
		relForeignKeys   []string
		relForeignFields []*schema.Field
		foreignFields    []*schema.Field
		foreignValues    [][]interface{}
		identityMap      = map[string][]reflect.Value{}
		inlineConds      []interface{}
	)

	db.Statement.Settings.Range(func(k, v interface{}) bool {
		tx.Statement.Settings.Store(k, v)
		return true
	})

	if rel.JoinTable != nil {
		var (
			joinForeignFields    = make([]*schema.Field, 0, len(rel.References))
			joinRelForeignFields = make([]*schema.Field, 0, len(rel.References))
			joinForeignKeys      = make([]string, 0, len(rel.References))
		)

		for _, ref := range rel.References {
			if ref.OwnPrimaryKey {
				joinForeignKeys = append(joinForeignKeys, ref.ForeignKey.DBName)
				joinForeignFields = append(joinForeignFields, ref.ForeignKey)
				foreignFields = append(foreignFields, ref.PrimaryKey)
			} else if ref.PrimaryValue != "" {
				tx = tx.Where(clause.Eq{Column: ref.ForeignKey.DBName, Value: ref.PrimaryValue})
			} else {
				joinRelForeignFields = append(joinRelForeignFields, ref.ForeignKey)
				relForeignKeys = append(relForeignKeys, ref.PrimaryKey.DBName)
				relForeignFields = append(relForeignFields, ref.PrimaryKey)
			}
		}

		joinIdentityMap, joinForeignValues := schema.GetIdentityFieldValuesMap(reflectValue, foreignFields)
		if len(joinForeignValues) == 0 {
			return
		}

		joinResults := rel.JoinTable.MakeSlice().Elem()
		column, values := schema.ToQueryValues(clause.CurrentTable, joinForeignKeys, joinForeignValues)
		db.AddError(tx.Where(clause.IN{Column: column, Values: values}).Find(joinResults.Addr().Interface()).Error)

		// convert join identity map to relation identity map
		fieldValues := make([]interface{}, len(joinForeignFields))
		joinFieldValues := make([]interface{}, len(joinRelForeignFields))
		for i := 0; i < joinResults.Len(); i++ {
			for idx, field := range joinForeignFields {
				fieldValues[idx], _ = field.ValueOf(joinResults.Index(i))
			}

			for idx, field := range joinRelForeignFields {
				joinFieldValues[idx], _ = field.ValueOf(joinResults.Index(i))
			}

			if results, ok := joinIdentityMap[utils.ToStringKey(fieldValues...)]; ok {
				joinKey := utils.ToStringKey(joinFieldValues...)
				identityMap[joinKey] = append(identityMap[joinKey], results...)
			}
		}

		_, foreignValues = schema.GetIdentityFieldValuesMap(joinResults, joinRelForeignFields)
	} else {
		for _, ref := range rel.References {
			if ref.OwnPrimaryKey {
				relForeignKeys = append(relForeignKeys, ref.ForeignKey.DBName)
				relForeignFields = append(relForeignFields, ref.ForeignKey)
				foreignFields = append(foreignFields, ref.PrimaryKey)
			} else if ref.PrimaryValue != "" {
				tx = tx.Where(clause.Eq{Column: ref.ForeignKey.DBName, Value: ref.PrimaryValue})
			} else {
				relForeignKeys = append(relForeignKeys, ref.PrimaryKey.DBName)
				relForeignFields = append(relForeignFields, ref.PrimaryKey)
				foreignFields = append(foreignFields, ref.ForeignKey)
			}
		}

		identityMap, foreignValues = schema.GetIdentityFieldValuesMap(reflectValue, foreignFields)
		if len(foreignValues) == 0 {
			return
		}
	}

	// nested preload
	for p, pvs := range preloads {
		tx = tx.Preload(p, pvs...)
	}

	reflectResults := rel.FieldSchema.MakeSlice().Elem()
	column, values := schema.ToQueryValues(clause.CurrentTable, relForeignKeys, foreignValues)

	for _, cond := range conds {
		if fc, ok := cond.(func(*gorm.DB) *gorm.DB); ok {
			tx = fc(tx)
		} else {
			inlineConds = append(inlineConds, cond)
		}
	}

	db.AddError(tx.Where(clause.IN{Column: column, Values: values}).Find(reflectResults.Addr().Interface(), inlineConds...).Error)

	fieldValues := make([]interface{}, len(relForeignFields))

	// clean up old values before preloading
	switch reflectValue.Kind() {
	case reflect.Struct:
		switch rel.Type {
		case schema.HasMany, schema.Many2Many:
			rel.Field.Set(reflectValue, reflect.MakeSlice(rel.Field.IndirectFieldType, 0, 10).Interface())
		default:
			rel.Field.Set(reflectValue, reflect.New(rel.Field.FieldType).Interface())
		}
	case reflect.Slice, reflect.Array:
		for i := 0; i < reflectValue.Len(); i++ {
			switch rel.Type {
			case schema.HasMany, schema.Many2Many:
				rel.Field.Set(reflectValue.Index(i), reflect.MakeSlice(rel.Field.IndirectFieldType, 0, 10).Interface())
			default:
				rel.Field.Set(reflectValue.Index(i), reflect.New(rel.Field.FieldType).Interface())
			}
		}
	}

	for i := 0; i < reflectResults.Len(); i++ {
		elem := reflectResults.Index(i)
		for idx, field := range relForeignFields {
			fieldValues[idx], _ = field.ValueOf(elem)
		}

		for _, data := range identityMap[utils.ToStringKey(fieldValues...)] {
			reflectFieldValue := rel.Field.ReflectValueOf(data)
			if reflectFieldValue.Kind() == reflect.Ptr && reflectFieldValue.IsNil() {
				reflectFieldValue.Set(reflect.New(rel.Field.FieldType.Elem()))
			}

			reflectFieldValue = reflect.Indirect(reflectFieldValue)
			switch reflectFieldValue.Kind() {
			case reflect.Struct:
				rel.Field.Set(data, reflectResults.Index(i).Interface())
			case reflect.Slice, reflect.Array:
				if reflectFieldValue.Type().Elem().Kind() == reflect.Ptr {
					rel.Field.Set(data, reflect.Append(reflectFieldValue, elem).Interface())
				} else {
					rel.Field.Set(data, reflect.Append(reflectFieldValue, elem.Elem()).Interface())
				}
			}
		}
	}
}