Tags are a lightweight taxonomy system that is perfect for blogs. Tags allow you to apply multiple categories to a blog post and allow multiple posts to be related to one another outside their category. On my own blog I use tags to organize the posts, so that people interested in reading my posts about Flask need only look under the "Flask" tag and find all the relevant posts. As per the spec that we discussed in Chapter 1, Creating Your First Flask Application, each blog entry can have as few or as many tags as you want, so a post about Flask might be tagged with both Flask and Python. Similarly, each tag (for example, Python) can have multiple entries associated with it. In database parlance, this is called a many-to-many relationship.

In order to model this, we must first create a model to store tags. This model will store the names of tags we use, so after we've added a few tags the table might look something like the following one: