title: Flask/SQLAlchemy error: TypeError: Incompatible collection type: [model] is not list-like
body: <p>i have nasty problem in very simple application. I try many different ways, but still cant figure out what I'm doing wrong. </p>
  
  <p>I use <strong>Flask</strong> + <strong>flask.ext.sqlalchemy</strong>, also my model relationship is <strong>many-to-many</strong>. </p>
  
  <pre><code>authorship = db.Table('authorship',
      db.Column('author_id', db.Integer, db.ForeignKey('author.id')),
      db.Column('book_id', db.Integer, db.ForeignKey('book.id'))
  )
  
  
  class Book(db.Model):
      id = db.Column(db.Integer, primary_key=True)
      title = db.Column(db.String(80), unique=False)
      description = db.Column(db.String(), unique=False)
  
      authors = db.relationship('Author', secondary=authorship, backref=db.backref('books', lazy='dynamic'))
  
      def __init__(self, title, description, authors):
          self.title = title
          self.description = description
          self.authors = authors
  
      def __repr__(self):
          return self.title
  
  
  class Author(db.Model):
      id = db.Column(db.Integer, primary_key=True)
      name = db.Column(db.String(50))
  
      def __init__(self, name):
          self.name = name
  
      def __repr__(self):
          return self.name
  </code></pre>
  
  <p>No matter how i change my models and play with relationship i still have same error (Author or other model): </p>
  
  <pre><code>TypeError: Incompatible collection type: Author is not list-like
  </code></pre>
  
  <p>Error shows up when i try to add new book. </p>
  
  <p>I use these models with WTForms: </p>
  
  <pre><code>@app.route('/add_book', methods=['GET', 'POST'])
  def add_book():
      if not session.get('logged_in'):
          abort(401)
      form = AddBookForm(request.form)
      if request.method == 'POST' and form.validate():
          a = Author(form.author.data)
          b = Book(form.title.data, form.description.data, a)
          db.session.add(b)
          flash("book added")
      return render_template('add_book.html', form=form) 
  </code></pre>