disallow empty metadata dict in favour of null

bigchaindb/common/schema/transaction.yaml

@@ -245,6 +245,7 @@ definitions:
     - type: object
       description: |
         User provided transaction metadata. This field may be ``null`` or may
-        contain an object with freeform metadata.
+        contain an non empty object with freeform metadata.
       additionalProperties: true
+      minProperties: 1
     - type: 'null'

docs/server/source/schema/transaction.rst

@@ -301,7 +301,7 @@ Metadata
 --------
 
 User provided transaction metadata. This field may be ``null`` or may
-contain an object with freeform metadata.
+contain an non empty object with freeform metadata.
 
 
 

tests/common/test_schema.py

@@ -16,6 +16,16 @@ def test_validate_transaction_signed_transfer(signed_transfer_tx):
     validate_transaction_schema(signed_transfer_tx.to_dict())
 
 
+def test_validate_fails_metadata_empty_dict(create_tx):
+    create_tx.metadata = {'a': 1}
+    validate_transaction_schema(create_tx.to_dict())
+    create_tx.metadata = None
+    validate_transaction_schema(create_tx.to_dict())
+    create_tx.metadata = {}
+    with raises(SchemaValidationError):
+        validate_transaction_schema(create_tx.to_dict())
+
+
 def test_validation_fails():
     with raises(SchemaValidationError):
         validate_transaction_schema({})