Adds a unique column value validator

HISTORY.rst

@@ -1,5 +1,9 @@
 Changelog
 ---------
+
+- Adds a unique column value validator.
+  [msom]
+
 0.42.0 (2019-01-28)
 ~~~~~~~~~~~~~~~~~~~
 

onegov/form/tests/test_validators.py

@@ -1,8 +1,52 @@
+from onegov.core.orm import SessionManager
+from onegov.form.validators import UniqueColumnValue
 from onegov.form.validators import ValidPhoneNumber
 from pytest import raises
+from sqlalchemy import Column
+from sqlalchemy import Text
+from sqlalchemy.ext.declarative import declarative_base
 from wtforms.validators import ValidationError
 
 
+def test_unique_column_value_validator(postgres_dsn):
+    Base = declarative_base()
+
+    class Dummy(Base):
+        __tablename__ = 'dummies'
+        name = Column(Text, nullable=False, primary_key=True)
+
+    class Field(object):
+        def __init__(self, name, data):
+            self.name = name
+            self.data = data
+
+    class Request(object):
+        def __init__(self, session):
+            self.session = session
+
+    class Form(object):
+        def __init__(self, session):
+            self.request = Request(session)
+
+    mgr = SessionManager(postgres_dsn, Base)
+    mgr.bases.append(Base)
+    mgr.set_current_schema('foobar')
+    session = mgr.session()
+    session.add(Dummy(name='Alice'))
+
+    validator = UniqueColumnValue(Dummy)
+    form = Form(session)
+
+    with raises(RuntimeError):
+        validator(form, Field('id', 'Alice'))
+    with raises(ValidationError):
+        validator(form, Field('name', 'Alice'))
+    validator(form, Field('name', 'Bob'))
+
+    form.model = session.query(Dummy).first()
+    validator(form, Field('name', 'Alice'))
+
+
 def test_phone_number_validator():
 
     class Field(object):

onegov/form/validators.py

@@ -219,3 +219,37 @@ def __call__(self, form, field):
             )
             if not valid:
                 raise ValidationError(self.message)
+
+
+class UniqueColumnValue(object):
+    """ Test if the given table does not already have a value in the column
+    (identified by the field name).
+
+    If the form provides a model with such an attribute, we allow this
+    value, too.
+
+    Expects an :class:`wtforms.StringField` instance.
+
+    Usage::
+
+        username = StringField(validators=[UniqueColumnValue(User)])
+
+    """
+
+    def __init__(self, table):
+        self.table = table
+
+    def __call__(self, form, field):
+        if field.name not in self.table.__table__.columns:
+            raise RuntimeError("The field name must match a column!")
+
+        if hasattr(form, 'model'):
+            if hasattr(form.model, field.name):
+                if getattr(form.model, field.name) == field.data:
+                    return
+
+        column = getattr(self.table, field.name)
+        query = form.request.session.query(column)
+        query = query.filter(column == field.data)
+        if query.first():
+            raise ValidationError(_("This value already exists."))