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    <p>The first mistake I can see if in your models-- your association correctly has two foreign keys, but the foreign key for the tag is an <code>Integer</code> but your primary key within your <code>Tag</code> class is a <code>String</code>-- they should match. Apart from that the models look good.</p> <p>You also should be able to simplify your <code>create_product</code> function:</p> <pre><code>def create_product(title, description, image, link, price, expiration, tags): image = imagefolder + image tag_list = [] for tag in tags: tagcheck = Tag.query.filter_by(name=tag).first() if tagcheck is None: tag_list.append(Tag(tag)) else: tag_list.append(tagcheck) product = Product(title, description, image, link, price, datetime.utcnow(), expiration, tag_list) return product </code></pre> <p>This can easily be moved inside your <code>__init___</code> constructor. In your example you deal directly with the association table, but you don't have to at all-- just trust the ORM to do the correct thing, that's the beauty of SQLAlchemy.</p> <p>Here's an example of what your product model could look like:</p> <pre><code>class Product(db.Model): id = db.Column(db.Integer, primary_key=True) title = db.Column(db.String(128)) description = db.Column(db.Text) image = db.Column(db.String(64)) link = db.Column(db.String(256)) price = db.Column(db.Float()) timestamp = db.Column(db.DateTime) expiration = db.Column(db.String(6)) tags = db.relationship('Tag', secondary=product_tags, backref=db.backref('products', lazy='dynamic')) def __init__(self, title, description, image, link, price, timestamp, expiration, tags): self.title = title self.description = description self.image = image self.link = link self.price = price self.timestamp = timestamp self.expiration = expiration for tag in tags: tagcheck = Tag.query.filter_by(name=tag).first() if tagcheck is None: self.tags.append(Tag(tag)) else: self.tags.append(tagcheck) def __repr__(self): return '&lt;{}, {}&gt;'.format(self.title, ":".join([x.name for x in self.tags])) </code></pre> <p>And to test it out, first lets add a couple of tags to the system:</p> <pre><code>ta = Tag('cat') tb = Tag('dog') db.session.add_all([ta, tb]) db.session.commit() &gt;&gt;&gt; Tag.query.all() [&lt;Tag u'cat'&gt;, &lt;Tag u'dog'&gt;] </code></pre> <p>Now lets add a product that uses those tags, plus a new tag.</p> <pre><code>p = Product( 'title', 'description', 'image', 'link', 0.0, datetime.now(), 'expiry', ['dog','cat','horse'] ) db.session.add(p) db.session.commit() </code></pre> <p>When we create that product, the constructor takes each of those three string tags, and says "Hey, does a tag of this name already exist?" if so, it uses it, and if not, it creates a new tag by that name. SQLAlchemy is clever enough to know to add the new Tag into the session, and to commit it when the product is committed.</p> <pre><code>&gt;&gt;&gt; Tag.query.all() [&lt;Tag u'cat'&gt;, &lt;Tag u'dog'&gt;, &lt;Tag u'horse'&gt;] </code></pre> <p>Now lets find all products with the dog tag (assuming more products have been added).</p> <pre><code>&gt;&gt;&gt; tag = Tag.query.get('dog') &gt;&gt;&gt; products = tag.products &gt;&gt;&gt; [x.title for x in products] ['title','other','examples'] </code></pre> <p>Again, at no point am I touching the association table at all, there's no need. SQLAlchemy is saving us effort.</p>
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    1. COIf I put that in the Tag __init__ would the tag be created no matter what? If it's a duplicate tag I don't want it created, I just want the association created. Also, thanks for pointing out the integer/string issue.
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    2. COI tried the changes you suggested, but I got the error AttributeError: 'Table' object has no attribute 'append'. It is in reference to product_tags.append.
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    3. COI mean add it into the product init, you pass in the tags the product needs as strings, then the product init decides if it needs to create them or if it can reuse existing ones within the database. Also I believe I've fixed the append error, I had a variable conflict in my example.
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