diff --git a/src/teilchensammler_cli/models.py b/src/teilchensammler_cli/models.py
index 61e7a54..dc54235 100644
--- a/src/teilchensammler_cli/models.py
+++ b/src/teilchensammler_cli/models.py
@@ -1,17 +1,31 @@
 from datetime import datetime
 
 from tortoise import Model, fields
+from tortoise.validators import MinValueValidator
 
 
 class TimestampMixin:
-    created_at: fields.Field[datetime] = fields.DatetimeField(auto_now_add=True)
-    modified_at: fields.Field[datetime] = fields.DatetimeField(auto_now=True)
+    created_at: fields.Field[datetime] = fields.DatetimeField(
+        auto_now_add=True,
+    )
+    modified_at: fields.Field[datetime] = fields.DatetimeField(
+        auto_now=True,
+    )
 
 
 class Teilchen(TimestampMixin, Model):
-    name: fields.Field[str] = fields.TextField()
-    description: fields.Field[str] = fields.TextField()
-    number: fields.Field[int] = fields.IntField()  # TODO: validate values >= 0
+    name: fields.Field[str] = fields.TextField(
+        null=False,
+    )
+    description: fields.Field[str] = fields.TextField(
+        null=False,
+    )
+    number: fields.Field[int] = fields.IntField(
+        null=False,
+        validators=[
+            MinValueValidator(0),
+        ],
+    )
 
     tags: fields.ManyToManyRelation["Tag"] = fields.ManyToManyField(
         "models.Tag",
@@ -23,12 +37,16 @@ class Teilchen(TimestampMixin, Model):
 
 
 class Tag(TimestampMixin, Model):
-    name: fields.Field[str] = fields.CharField(max_length=20)
+    name: fields.Field[str] = fields.CharField(
+        null=False,
+        max_length=20,
+    )
 
 
 class TeilchenTag(Model):
     teilchen: fields.ForeignKeyRelation[Teilchen] = fields.ForeignKeyField(
-        "models.Teilchen"
+        "models.Teilchen",
+    )
+    tag: fields.ForeignKeyRelation[Tag] = fields.ForeignKeyField(
+        "models.Tag",
     )
-    tag: fields.ForeignKeyRelation[Tag] = fields.ForeignKeyField("models.Tag")
-    weight: fields.Field[int] = fields.IntField()  # range? percentage?