process inherited primary keys correctely

When using joined table inheritance (http://docs.sqlalchemy.org/en/latest/orm/inheritance.html), it is common practice to name the pk-property the same like the pk-property of the parent. The child-property ist a pk itself and has a foreign-key relationship to the pk-property of the parent.

Example:

    class BaseWahl(Model):
        __tablename__ = 'basewahlen'

        id = Column(Integer, primary_key=True, autoincrement=True)
        discriminator = db.Column(db.String(50))

        __mapper_args__ = {
        'polymorphic_identity':'basewahl',
        'polymorphic_on': discriminator
        }

    class Wahl(BaseWahl):
        __tablename__ = 'wahlen'

        id = Column(Integer, ForeignKey('basewahlen.id'), primary_key=True)
        __mapper_args__ = {
            'polymorphic_identity':'wahl',
        }

`AdminModelConverter.convert()` does not allow Properties with multiple columns, but will raise a `TypeError`. I changed it into the following way:

- If
  - more than 1 column for the property
  - all columns are primary keys
  - only *one* does not have a Foreign key
  - only one column corresponds to the current model
- select the column, that corresponds to the current model

I applied the same code to `ModelView.scaffold_list_columns()` and extended it, the primary key in this special constellation actually is *not* ignored, even if it has a foreign key property, so it can be displayed using `column_display_pk = True`.

This solution actually works for me. I do not have enough insight into *Flask-Admin* and definitely even less into *sqlalchemy* to think my solution is the correct one for every situation, but it may be something to think about.

flask_admin/contrib/sqla/form.py

@@ -132,10 +132,22 @@ class AdminModelConverter(ModelConverterBase):
             if hasattr(prop, 'columns'):
                 # Check if more than one column mapped to the property
                 if len(prop.columns) != 1:
-                    raise TypeError('Can not convert multiple-column properties (%s.%s)' % (model, prop.key))
-
-                # Grab column
-                column = prop.columns[0]
+                    # Check if all columns are primary keys and _one_ does not have a foreign key -> looks like joined
+                    # table inheritance: http://docs.sqlalchemy.org/en/latest/orm/inheritance.html with "standard
+                    # practice" of same column name
+                    if len([column for column in prop.columns if column.primary_key]) == len(prop.columns) and \
+                            len([column for column in prop.columns if column.foreign_keys]) == len(prop.columns)-1:
+                        # Get the column(s) of the current model - should always be only one, I think (but not sure)
+                        candidates = [column for column in prop.columns if column.expression == prop.expression]
+                        if len(candidates) != 1:
+                            raise TypeError('Can not convert multiple-column pk-Property (%s.%s)' % (model, prop.key))
+                        else:
+                            column = candidates[0]
+                    else:
+                        raise TypeError('Can not convert multiple-column properties (%s.%s)' % (model, prop.key))
+                else:
+                    # Grab column
+                    column = prop.columns[0]
 
                 # Do not display foreign keys - use relations
                 if column.foreign_keys:

flask_admin/contrib/sqla/view.py

@@ -308,10 +308,27 @@ class ModelView(BaseModelView):
                 if self.column_display_all_relations or p.direction.name == 'MANYTOONE':
                     columns.append(p.key)
             elif hasattr(p, 'columns'):
-                # TODO: Check for multiple columns
-                column = p.columns[0]
+                column_inherited_primary_key = False
+                if len(p.columns) != 1:
+                    # Check if all columns are primary keys and _one_ does not have a foreign key -> looks like joined
+                    # table inheritance: http://docs.sqlalchemy.org/en/latest/orm/inheritance.html with "standard
+                    # practice" of same column name
+                    if len([column for column in p.columns if column.primary_key]) == len(p.columns) and \
+                            len([column for column in p.columns if column.foreign_keys]) == len(p.columns)-1:
+                        # Get the column(s) of the current model - should always be only one, I think (but not sure)
+                        candidates = [column for column in p.columns if column.expression == p.expression]
+                        if len(candidates) != 1:
+                            raise TypeError('Can not convert multiple-column pk-Property (%s.%s)' % (model, p.key))
+                        else:
+                            column = candidates[0]
+                            column_inherited_primary_key = True
+                    else:
+                        raise TypeError('Can not convert multiple-column properties (%s.%s)' % (model, p.key))
+                else:
+                    # Grab column
+                    column = p.columns[0]
 
-                if column.foreign_keys:
+                if column.foreign_keys and not column_inherited_primary_key:
                     continue
 
                 if not self.column_display_pk and column.primary_key: