“Validate” in serializers.ModelSerializer in Django REST API doesn't work

I have some validation in my modelserializer that doesn't work: here is my serializer class:

from rest_framework import serializers
from ..models import Status

class StatusSerializer(serializers.ModelSerializer):
    class Meta:
        model=Status
        fields=[
            'user',
            'content',
            'image',
        ]

    def validate_content(self,value):
        if len(value)>5:
            raise serializers.ValidationError('Content is too long')
        return value


    def validate(self,data):
        content=data.get('content',None)
        if content=="":
            content=None
        image=data.get('image',None)
        if content is None and image is None:
            raise serializers.ValidationError('content or image is requiered')
        return data

here is where I try to make an entry, and I expect the result to be False, but it comes up as True

data ={'user':1}
serializer=StatusSerializer(data=data)
serializer.is_valid()

True

data ={'user':1,'content':'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'}
serializer=StatusSerializer(data=data)
serializer.is_valid()

True

here is my model just in case:

    class Status(models.Model):
        user=models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
        content = models.TextField(null=True, blank=True)
        image = models.ImageField(upload_to=upload_update_image,null=True, blank=True)
        timestamp = models.DateTimeField(auto_now_add=True)
        updated = models.DateTimeField(auto_now_add=True)

        objects = UpdateManager()

        def __str__(self):
            return str(self.content)[:50]

        class Meta:
            verbose_name='Status post'
            verbose_name_plural='Status posts'
            app_label='status'

I appreciate if any one give me a hint of what I am doing wrong.