update hyperlinkrelated to gracefully support nullable / non-required…

… sqla relations

flask_marshmallow/sqla.py

@@ -8,6 +8,7 @@
     from Flask-SQLALchemy.
 """
 from flask import url_for, current_app
+from werkzeug.routing import BuildError
 from six.moves.urllib import parse
 
 import marshmallow_sqlalchemy as msqla
@@ -62,7 +63,13 @@ def __init__(self, endpoint, url_key='id', external=False, **kwargs):
     def _serialize(self, value, attr, obj):
         key = super(HyperlinkRelated, self)._serialize(value, attr, obj)
         kwargs = {self.url_key: key}
-        return url_for(self.endpoint, _external=self.external, **kwargs)
+        try:
+            return url_for(self.endpoint, _external=self.external, **kwargs)
+        except BuildError as e:
+            if not self.required:
+                return self.missing if self.missing else None
+            else:
+                raise e
 
     def _deserialize(self, value, *args, **kwargs):
         if self.external:

tests/test_sqla.py

@@ -3,6 +3,7 @@
 from flask import Flask, url_for
 from flask_sqlalchemy import SQLAlchemy
 from werkzeug.wrappers import BaseResponse
+from werkzeug.routing import BuildError
 
 from flask_marshmallow import Marshmallow
 from flask_marshmallow.sqla import HyperlinkRelated
@@ -138,12 +139,16 @@ class Meta:
         deserialized, errors = get_load_data(book_schema, book_result)
         assert deserialized.author == author
 
+        # Deserialization works on nullable / non-required relation
+        book.author = None
+        book_result = book_schema.dump(book)
+        assert book_result['author'] is None
+
     def test_hyperlink_related_field_errors(self, extma, models, db, extapp):
         class BookSchema(extma.ModelSchema):
             class Meta:
                 model = models.Book
-
-            author = HyperlinkRelated('author')
+            author = HyperlinkRelated('author', required=True)
 
         book_schema = BookSchema()
 
@@ -166,6 +171,12 @@ class Meta:
         deserialized, errors = get_load_data(book_schema, book_result)
         assert 'URL pattern "pk" not found' in errors['author'][0]
 
+        # Deserialization fails on empty required field
+        book.author = None
+        with pytest.raises(BuildError) as exc:
+            book_schema.dump(book)
+        assert "endpoint 'author'" in str(exc.value)
+
     def test_hyperlink_related_field_external(self, extma, models, db, extapp):
         class BookSchema(extma.ModelSchema):
             class Meta: