check response input is json

api_reflector/admin.py

@@ -1,16 +1,36 @@
 """
 Declares the flask-admin instance and sets up the model views.
 """
+import json
+from json import JSONDecodeError
+
 from flask import redirect, url_for
 from flask_admin import Admin, AdminIndexView
 from flask_admin.contrib.sqla import ModelView
 from flask_admin.menu import MenuLink
 from jinja2.runtime import Context
 from slugify import slugify
+from wtforms import validators
 
 from api_reflector import auth, db, models
 
 
+def is_valid_json(data: str) -> None:
+    """
+    Validator function for checking whether form field content is a valid JSON.
+    """
+
+    try:
+        json.loads(data)
+    except JSONDecodeError as exc:
+        raise validators.ValidationError("Response content data must be a valid JSON object or empty.") from exc
+
+
+form_validators = {
+    "application/json": is_valid_json,
+}
+
+
 def admin_view(model: db.Model):
     """
     Registers a model with the admin.
@@ -97,6 +117,11 @@ class ResponseView(RestrictedView):
     form_widget_args = {"content": {"rows": 8, "style": "font-family: monospace;"}}
     column_searchable_list = ("name",)
 
+    def on_model_change(self, form, model, is_created):
+        if validator := form_validators.get(form.content_type.data):
+            validator(form.content.data)
+            super().on_model_change(form, model, is_created)
+
     def content_formatter(self, _ctx: Context, model: models.Model, _name: str):
         """
         Limits the content field to a maximum length in the list view.

api_reflector/models.py

@@ -74,7 +74,7 @@ class Response(Model):
 
     status_code = Column(Integer, nullable=False, default=200)
     content_type = Column(String, nullable=False, default="application/json")
-    content = Column(Text, nullable=False, default="")
+    content = Column(Text, nullable=False, default="{}")
 
     is_active = Column(Boolean, nullable=False, default=True)