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    <h2>The cause of the problem</h2> <p>The crux of your problem is that <strong>SQL Alchemy is expecting the <code>permissions</code> relationship to be populated with sqlalchemy <code>permission</code> objects, not the <code>int</code>s you are passing it</strong>.<br> In your view: </p> <pre><code>form = EditUser(request.form, obj=user) form.permissions.choices = [(p.id, p.perm) for p in Permission.query.order_by('perm')] form.permissions.data = [p.id for p in user.permissions] </code></pre> <p>So in your form, <code>form.permissions</code> is actually just holding the <code>id</code> (int) of the permission object's, not the objects themselves. When you call <code>form.populate_obj(user)</code>, when it gets to the permissions segment, its going to call this method: </p> <p><code>setattr(obj, name, self.data)</code> </p> <p>Which translates to: </p> <p><code>setattr(user, permissions, sefl.data)</code> </p> <p>Which translates to: </p> <p><code>user.permissions = [1, 3, 4 ...]</code> </p> <p>But SQL Alchemy expects: </p> <p><code>user.permissions = [sqlalchemyobject1, sqlalchemyobject2, ...]</code> </p> <h2>Solution</h2> <p>There are at least a few ways to solve this problem. The method I use is sqlalchemy's <a href="http://docs.sqlalchemy.org/en/rel_0_7/orm/extensions/associationproxy.html">Association Proxy</a>. This will let you do things like pass a list of integers to your user model, and <strong>have the Model convert them to Permissions objects</strong>: </p> <pre><code>class User(db.Model): ... permissions_relationship = ('Permission', cascade="all,delete,delete-orphan") permissions = association_proxy('permissions_relationship', 'perm', creator=lambda perm: Permission(perm=perm) class Permission(db.Model): ... perm = ... def __init__(self,perm): self.perm = perm </code></pre> <p>In a Nutshell, the <code>association_proxy</code> call simply says: "When you get passed a list of items (Ints, Strings, whatever), convert them to <code>Permission</code> objects by doing this: <code>Permission(perm=perm)</code>. Using this solution in your above code, you would remove the line <code>form.permissions.data = [p.id for p in user.permissions]</code>. </p> <p>Finally, <code>wtforms</code> also provides a <a href="http://wtforms.simplecodes.com/docs/1.0.2/ext.html#wtforms.ext.sqlalchemy.fields.QuerySelectMultipleField">QuerySelectMultiple field</a>. I haven't used it, but imagine it would also serve as a solution to this problem. </p>
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    1. POHow I do to update data on many-to-many with WTForms and SQLAlchemy?
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    1. COAmazingly answers, thanks a lot !
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