diff --git a/models/migrations/v15.go b/models/migrations/v15.go
index e29067c98..3492a7190 100644
--- a/models/migrations/v15.go
+++ b/models/migrations/v15.go
@@ -10,21 +10,15 @@ import (
 	"github.com/go-xorm/xorm"
 )
 
-// UserV15 describes the added field for User
-type UserV15 struct {
-	KeepEmailPrivate        bool
-	AllowCreateOrganization bool
-}
-
-// TableName will be invoked by XORM to customrize the table name
-func (*UserV15) TableName() string {
-	return "user"
-}
-
 func createAllowCreateOrganizationColumn(x *xorm.Engine) error {
-	if err := x.Sync2(new(UserV15)); err != nil {
+	type User struct {
+		KeepEmailPrivate        bool
+		AllowCreateOrganization bool
+	}
+
+	if err := x.Sync2(new(User)); err != nil {
 		return fmt.Errorf("Sync2: %v", err)
-	} else if _, err = x.Where("type=0").Cols("allow_create_organization").Update(&UserV15{AllowCreateOrganization: true}); err != nil {
+	} else if _, err = x.Where("`type` = 0").Cols("allow_create_organization").Update(&User{AllowCreateOrganization: true}); err != nil {
 		return fmt.Errorf("set allow_create_organization: %v", err)
 	}
 	return nil
diff --git a/models/migrations/v16.go b/models/migrations/v16.go
index 8ce37ea7d..5b8ec19d3 100644
--- a/models/migrations/v16.go
+++ b/models/migrations/v16.go
@@ -13,17 +13,6 @@ import (
 	"github.com/go-xorm/xorm"
 )
 
-// RepoUnit describes all units of a repository
-type RepoUnit struct {
-	ID          int64
-	RepoID      int64 `xorm:"INDEX(s)"`
-	Type        int   `xorm:"INDEX(s)"`
-	Index       int
-	Config      map[string]interface{} `xorm:"JSON"`
-	CreatedUnix int64                  `xorm:"INDEX CREATED"`
-	Created     time.Time              `xorm:"-"`
-}
-
 // Enumerate all the unit types
 const (
 	V16UnitTypeCode            = iota + 1 // 1 code
@@ -37,14 +26,25 @@ const (
 	V16UnitTypeExternalTracker            // 9 ExternalTracker
 )
 
-// Repo describes a repository
-type Repo struct {
-	ID                                                                               int64
-	EnableWiki, EnableExternalWiki, EnableIssues, EnableExternalTracker, EnablePulls bool
-	ExternalWikiURL, ExternalTrackerURL, ExternalTrackerFormat, ExternalTrackerStyle string
-}
-
 func addUnitsToTables(x *xorm.Engine) error {
+	// RepoUnit describes all units of a repository
+	type RepoUnit struct {
+		ID          int64
+		RepoID      int64 `xorm:"INDEX(s)"`
+		Type        int   `xorm:"INDEX(s)"`
+		Index       int
+		Config      map[string]interface{} `xorm:"JSON"`
+		CreatedUnix int64                  `xorm:"INDEX CREATED"`
+		Created     time.Time              `xorm:"-"`
+	}
+
+	// Repo describes a repository
+	type Repo struct {
+		ID                                                                               int64
+		EnableWiki, EnableExternalWiki, EnableIssues, EnableExternalTracker, EnablePulls bool
+		ExternalWikiURL, ExternalTrackerURL, ExternalTrackerFormat, ExternalTrackerStyle string
+	}
+
 	var repos []Repo
 	err := x.Table("repository").Select("*").Find(&repos)
 	if err != nil {
diff --git a/models/migrations/v37.go b/models/migrations/v37.go
index aac00e84c..00653a780 100644
--- a/models/migrations/v37.go
+++ b/models/migrations/v37.go
@@ -7,16 +7,19 @@ package migrations
 import (
 	"html"
 
-	"code.gitea.io/gitea/models"
-
 	"github.com/go-xorm/xorm"
 )
 
 func unescapeUserFullNames(x *xorm.Engine) (err error) {
+	type User struct {
+		ID       int64 `xorm:"pk autoincr"`
+		FullName string
+	}
+
 	const batchSize = 100
 	for start := 0; ; start += batchSize {
-		users := make([]*models.User, 0, batchSize)
-		if err := x.Limit(start, batchSize).Find(users); err != nil {
+		users := make([]*User, 0, batchSize)
+		if err := x.Limit(batchSize, start).Find(&users); err != nil {
 			return err
 		}
 		if len(users) == 0 {
@@ -24,7 +27,7 @@ func unescapeUserFullNames(x *xorm.Engine) (err error) {
 		}
 		for _, user := range users {
 			user.FullName = html.UnescapeString(user.FullName)
-			if _, err := x.Cols("full_name").Update(user); err != nil {
+			if _, err := x.ID(user.ID).Cols("full_name").Update(user); err != nil {
 				return err
 			}
 		}
diff --git a/models/migrations/v38.go b/models/migrations/v38.go
index c85688db7..6f66484b0 100644
--- a/models/migrations/v38.go
+++ b/models/migrations/v38.go
@@ -5,12 +5,26 @@
 package migrations
 
 import (
+	"time"
+
 	"code.gitea.io/gitea/models"
 
+	"github.com/go-xorm/core"
 	"github.com/go-xorm/xorm"
 )
 
 func removeCommitsUnitType(x *xorm.Engine) (err error) {
+	// RepoUnit describes all units of a repository
+	type RepoUnit struct {
+		ID          int64
+		RepoID      int64 `xorm:"INDEX(s)"`
+		Type        int   `xorm:"INDEX(s)"`
+		Index       int
+		Config      core.Conversion `xorm:"TEXT"`
+		CreatedUnix int64           `xorm:"INDEX CREATED"`
+		Created     time.Time       `xorm:"-"`
+	}
+
 	// Update team unit types
 	const batchSize = 100
 	for start := 0; ; start += batchSize {
diff --git a/models/migrations/v39.go b/models/migrations/v39.go
index 41279484e..95ae0c96a 100644
--- a/models/migrations/v39.go
+++ b/models/migrations/v39.go
@@ -13,26 +13,37 @@ import (
 	"github.com/go-xorm/xorm"
 )
 
-// Stopwatch see models/issue_stopwatch.go
-type Stopwatch struct {
-	ID          int64     `xorm:"pk autoincr"`
-	IssueID     int64     `xorm:"INDEX"`
-	UserID      int64     `xorm:"INDEX"`
-	Created     time.Time `xorm:"-"`
-	CreatedUnix int64
-}
-
-// TrackedTime see models/issue_tracked_time.go
-type TrackedTime struct {
-	ID          int64     `xorm:"pk autoincr" json:"id"`
-	IssueID     int64     `xorm:"INDEX" json:"issue_id"`
-	UserID      int64     `xorm:"INDEX" json:"user_id"`
-	Created     time.Time `xorm:"-" json:"created"`
-	CreatedUnix int64     `json:"-"`
-	Time        int64     `json:"time"`
-}
-
 func addTimetracking(x *xorm.Engine) error {
+	// RepoUnit describes all units of a repository
+	type RepoUnit struct {
+		ID          int64
+		RepoID      int64 `xorm:"INDEX(s)"`
+		Type        int   `xorm:"INDEX(s)"`
+		Index       int
+		Config      map[string]interface{} `xorm:"JSON"`
+		CreatedUnix int64                  `xorm:"INDEX CREATED"`
+		Created     time.Time              `xorm:"-"`
+	}
+
+	// Stopwatch see models/issue_stopwatch.go
+	type Stopwatch struct {
+		ID          int64     `xorm:"pk autoincr"`
+		IssueID     int64     `xorm:"INDEX"`
+		UserID      int64     `xorm:"INDEX"`
+		Created     time.Time `xorm:"-"`
+		CreatedUnix int64
+	}
+
+	// TrackedTime see models/issue_tracked_time.go
+	type TrackedTime struct {
+		ID          int64     `xorm:"pk autoincr" json:"id"`
+		IssueID     int64     `xorm:"INDEX" json:"issue_id"`
+		UserID      int64     `xorm:"INDEX" json:"user_id"`
+		Created     time.Time `xorm:"-" json:"created"`
+		CreatedUnix int64     `json:"-"`
+		Time        int64     `json:"time"`
+	}
+
 	if err := x.Sync2(new(Stopwatch)); err != nil {
 		return fmt.Errorf("Sync2: %v", err)
 	}
@@ -40,25 +51,23 @@ func addTimetracking(x *xorm.Engine) error {
 		return fmt.Errorf("Sync2: %v", err)
 	}
 	//Updating existing issue units
-	var units []*RepoUnit
-	x.Where("type = ?", V16UnitTypeIssues).Find(&units)
+	units := make([]*RepoUnit, 0, 100)
+	err := x.Where("`type` = ?", V16UnitTypeIssues).Find(&units)
+	if err != nil {
+		return fmt.Errorf("Query repo units: %v", err)
+	}
 	for _, unit := range units {
 		if unit.Config == nil {
 			unit.Config = make(map[string]interface{})
 		}
-		changes := false
 		if _, ok := unit.Config["EnableTimetracker"]; !ok {
 			unit.Config["EnableTimetracker"] = setting.Service.DefaultEnableTimetracking
-			changes = true
 		}
 		if _, ok := unit.Config["AllowOnlyContributorsToTrackTime"]; !ok {
 			unit.Config["AllowOnlyContributorsToTrackTime"] = setting.Service.DefaultAllowOnlyContributorsToTrackTime
-			changes = true
 		}
-		if changes {
-			if _, err := x.ID(unit.ID).Cols("config").Update(unit); err != nil {
-				return err
-			}
+		if _, err := x.ID(unit.ID).Cols("config").Update(unit); err != nil {
+			return err
 		}
 	}
 	return nil