Question

I just started with Flask and SQLAlchemy in flask.

So I have a many-to-many relationship using the example here http://docs.sqlalchemy.org/en/latest/orm/tutorial.html
If you scrolldown to the part about Keywords and tags this is what I am working on.
So far I am able to insert new Keywords related to my Post and I am using append. Which is wrong I know. So what happens is that the next time a non unique keyword occurs in a blog post it will throw an error about Conflict with Keyword (since keywords are supposed to be unique)

I know the right way is something else, I just don’t know what. I have seen an example of
get_or_create(keyword) which basically filters by keyword and then adds it if not found. However I believe as data size grows this will also be wrong. (Several calls on every save with single insert). I love the way SQLAlchemy is doing multiple insert automatically. I wish to keep that but avoid this duplicate key issue.

Edit: found the solution, SQLAlchemy docs guide you towards error but the explanation is in there. I have added the answer.

Answer 1

Ok after hours of trial and error I found the solution, plus somethings I was doing wrong.

This is how SQL alchemy works. the answer is merge.

make a LIST of tags as Tag models, don;t matter if they exist as long as your primary key is name or something unique.

tags = [Tag('a1'),Tag('a2')]

Say you have Tag a1 already in DB but we don’t really care. All we want is to insert if related data does not exist. WHich is what SQLalchemy does.
Now you make a Post with the LIST of ALL the tags we made. If its one only , it also is a list.

therefore

new_post = Post('a great new post',post_tags=tags)
db.session.merge(new_post)
db.session.commit()

I have used Flask syntax but the idea is same. Just make sure you are not creating the Model OUTSIDE the session. More likely, you wont do it.

This was actually simple but nowhere in the SQLAlchemy docs this example is mentioned. They use append() which is wrong. It’s only to create new Tags knowing you are not making duplicates.

Hope it helps.