SQLite database for a micro/tumble blog application
Everything is good by this point but I'd change the relation between posts and tags from one-to-many to many-to-many.

You can do this with this code:

tags_to_posts = db.Table(
    "tags_and_posts",
    db.Column( "post_id", db.Integer, db.ForeignKey( "posts.id" ) ),
    db.Column( "tag_id", db.Integer, db.ForeignKey( "tags.id" ) )
)

class Post(db.Model):
    # ...
    tags = db.relationship(
        "Tag",
        secondary=tags_and_posts,
        backref=db.backref( "posts", lazy="dynamic" ),
        passive_deletes=True,
    )


Then tags will be just a list. Assuming my_tag and my_post exist, you'll be able to do something like this:

# Add tag to post
my_post.tags.append(my_tag)
db.session.add(my_post)
db.session.add(my_tag)
db.session.commit()

# Remove tag from post
# You need to be sure that my_tag is in my_post.tags
my_post.tags.remove(my_tag)
db.session.add(my_post)
db.session.add(my_tag)
db.session.commit()


I hope, it helps.