add example for filtering options in forms

examples/sqla-filter-selectable/README.rst

+SQLA backend example showing how to filter select dropdown options in forms.
+
+To run this example:
+
+1. Clone the repository::
+
+    git clone https://github.com/flask-admin/flask-admin.git
+    cd flask-admin
+
+2. Create and activate a virtual environment::
+
+    virtualenv env
+    source env/bin/activate
+
+3. Install requirements::
+
+    pip install -r 'examples/sqla-filter-selectable/requirements.txt'
+
+4. Run the application::
+
+    python examples/sqla-filter-selectable/app.py

examples/sqla-filter-selectable/app.py

+from flask import Flask
+from flask_sqlalchemy import SQLAlchemy
+
+import flask_admin as admin
+from flask_admin.contrib import sqla
+
+# Create application
+app = Flask(__name__)
+
+# Create dummy secrey key so we can use sessions
+app.config['SECRET_KEY'] = '123456790'
+
+# Create in-memory database
+app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///sample_db_3.sqlite'
+app.config['SQLALCHEMY_ECHO'] = True
+db = SQLAlchemy(app)
+
+
+# Flask views
+@app.route('/')
+def index():
+    return '<a href="/admin/">Click me to get to Admin!</a>'
+
+
+class Person(db.Model):
+    id = db.Column(db.Integer, primary_key=True)
+    name = db.Column(db.String(50))
+    pets = db.relationship('Pet', backref='person')
+
+    def __unicode__(self):
+        return self.name
+
+
+class Pet(db.Model):
+    id = db.Column(db.Integer, primary_key=True)
+    name = db.Column(db.String(50))
+    person_id = db.Column(db.Integer, db.ForeignKey('person.id'))
+    available = db.Column(db.Boolean)
+
+    def __unicode__(self):
+        return self.name
+
+
+class PersonAdmin(sqla.ModelView):
+    """ Override ModelView to filter options available in forms. """
+
+    def create_form(self):
+        return self._use_filtered_parent(
+            super(PersonAdmin, self).create_form()
+        )
+
+    def edit_form(self, obj):
+        return self._use_filtered_parent(
+            super(PersonAdmin, self).edit_form(obj)
+        )
+
+    def _use_filtered_parent(self, form):
+        form.pets.query_factory = self._get_parent_list
+        return form
+
+    def _get_parent_list(self):
+        # only show available pets in the form
+        return Pet.query.filter_by(available=True).all()
+
+    def __unicode__(self):
+        return self.name
+
+
+# Create admin
+admin = admin.Admin(app, name='Example: SQLAlchemy - Filtered Form Selectable',
+                    template_mode='bootstrap3')
+admin.add_view(PersonAdmin(Person, db.session))
+admin.add_view(sqla.ModelView(Pet, db.session))
+
+if __name__ == '__main__':
+    # Recreate DB
+    db.drop_all()
+    db.create_all()
+
+    person = Person(name='Bill')
+    pet1 = Pet(name='Dog', available=True)
+    pet2 = Pet(name='Fish', available=True)
+    pet3 = Pet(name='Ocelot', available=False)
+    db.session.add_all([person, pet1, pet2, pet3])
+    db.session.commit()
+
+    # Start app
+    app.run(debug=True)

examples/sqla-filter-selectable/requirement.txt

+Flask
+Flask-Admin
+Flask-SQLAlchemy