diff --git a/file/model.go b/file/model.go
index e855e79..bbd1bb7 100644
--- a/file/model.go
+++ b/file/model.go
@@ -2,14 +2,16 @@ package file
 
 import (
 	"github.com/jinzhu/gorm"
+	"time"
 )
 
 type File struct {
 	gorm.Model
 	Name        string
-	Path        string
+	Path        string `gorm:"not null"`
 	Type        string
 	Size        uint
-	Dimmensions string
-	FileUser    User
+	Dimmensions string // TODO: Mixed Type
+	FileUser    User   `gorm:"not null"`
+	Date        Time   `gorm:"default:Time.Now"`
 }
diff --git a/project/model.go b/project/model.go
index 46174b5..e154892 100644
--- a/project/model.go
+++ b/project/model.go
@@ -8,10 +8,10 @@ import (
 
 type Project struct {
 	gorm.Model
-	Name              string
-	UserProject       User            // XXX: association?
-	Visualizations    []Visualization // TODO: association & foreign key
-	SimulationProject Simulation      // XXX: association?n
+	Name              string `gorm:"not null"`
+	ProjectUser       User   `gorm:"not null"`
+	Visualizations    []Visualization
+	ProjectSimulation Simulation `gorm:"not null"`
 }
 
 // TODO: execute before project.delete()
diff --git a/simulation/model.go b/simulation/model.go
index 6b7bf45..066eae6 100644
--- a/simulation/model.go
+++ b/simulation/model.go
@@ -8,10 +8,10 @@ import (
 
 type Simulation struct {
 	gorm.Model
-	Name            string
-	Running         bool              `gorm:"default:false"`
-	Models          []SimulationModel // TODO: association & foreign key
-	Projects        []Project         // TODO: association & foreign key
-	User            []Users           // TODO: association & foreign key
-	StartParameters []string          // TODO: Mixed Type
+	Name            string `gorm:"not null"`
+	Running         bool   `gorm:"default:false"`
+	Models          []SimulationModel
+	Projects        []Project
+	SimulationUser  User     `gorm:"not null"`
+	StartParameters []string // TODO: Mixed Type
 }
diff --git a/simulationModel/model.go b/simulationModel/model.go
index b6872dc..4e1d2ba 100644
--- a/simulationModel/model.go
+++ b/simulationModel/model.go
@@ -13,6 +13,6 @@ type SimulationModel struct {
 	OutputMapping   []string   // TODO: Mixed Type
 	InputMapping    []string   // TODO: Mixed Type
 	StartParameters []string   // TODO: Mixed Type
-	ModelSimulation Simulation // TODO: association & foreign key
-	ModelSimulator  Simulator  // TODO: association & foreign key
+	ModelSimulation Simulation `gorm:"not null"`
+	ModelSimulator  Simulator  `gorm:"not null"`
 }
diff --git a/simulator/model.go b/simulator/model.go
index 02d1b94..1903ae9 100644
--- a/simulator/model.go
+++ b/simulator/model.go
@@ -2,15 +2,17 @@ package simulator
 
 import (
 	"github.com/jinzhu/gorm"
+	"time"
 )
 
 type Simulator struct {
 	gorm.Model
-	UUID          string
-	Host          string `gorm:"default:"`
-	Model         string `gorm:"default:"`
-	Uptime        int    `gorm:"default:0"`
-	State         string `gorm:"default:"`
-	Properties    []string
-	RawProperties []string
+	UUID          string   `gorm:"unique;not null"`
+	Host          string   `gorm:"default:"`
+	Model         string   `gorm:"default:"`
+	Uptime        int      `gorm:"default:0"`
+	State         string   `gorm:"default:"`
+	StateUpdateAt Time     `gorm:"default:Time.Now"`
+	Properties    []string // TODO: mixed type
+	RawProperties []string // TODO: mixed type
 }
diff --git a/user/model.go b/user/model.go
index 7045f1a..7047a6e 100644
--- a/user/model.go
+++ b/user/model.go
@@ -10,13 +10,13 @@ import (
 
 type User struct {
 	gorm.Model
-	Username    string `gorm:"unique"`
-	Password    string
-	Mail        string      `gorm:"default:"`
-	Role        string      `gorm:"default:user"`
-	Projects    []Project   // TODO: association & foreign key
-	Simulations []Simulaion // TODO: association & foreign key
-	Files       []File      // TODO: association & foreign key
+	Username    string `gorm:"unique;not null"`
+	Password    string `gorm:"not null"`
+	Mail        string `gorm:"default:"`
+	Role        string `gorm:"default:user"`
+	Projects    []Project
+	Simulations []Simulaion
+	Files       []File
 }
 
 // TODO: callback for verifying password
diff --git a/visualization/model.go b/visualization/model.go
index 9e64522..9226441 100644
--- a/visualization/model.go
+++ b/visualization/model.go
@@ -6,9 +6,9 @@ import (
 
 type Visualization struct {
 	gorm.Model
-	Name                 string
-	VisualizationProject Project
-	Widgets              string
-	Grid                 int `gorm:"default:15"`
-	VisualizationUser    User
+	Name                 string   `gorm:"not null"`
+	VisualizationProject Project  `gorm:"not null"`
+	Widgets              []string // XXX: array of what type?
+	Grid                 int      `gorm:"default:15"`
+	VisualizationUser    User     `gorm:"not null"`
 }