Week 3 — Tag Support
adding tags
Tags are usually single words that describe entries in a brief way. Although it is possible to use tags that include whitespace or are very long, it is not recommended to do so. To add new tags to an entry, use the method tag from the Database class. It receives an instance of DatabaseEntry (the entry that shall get assigned new tags) and at least one tag name. The new tags are stored in the attribute tags of this modified database entry. This is a list of Tag instances which contain a unique ID and the name of the tag (which is also unique). The Database class has also an attribute called tags. This stores all tags that have been saved in the database.
In the following example, you can see that the attributes tags of both the database object and of the entry are by default empty lists. After the created entry is tagged with the tags “shiny”, “bright”, and “yellow”, the tags attribute of entry is not empty anymore but contains a list of Tag instances, as expected. But why is the list database.tags still empty? The answer: because the entry is not saved in the database yet. It must first be added with the add method. After that, the tags are saved in the database as we can see in the ID information (if a database-related object such as instances of DatabaseEntry or Tag does not have an ID, it is not stored in the database).
>>> from pprint import pprint
>>> from sunpy.database import Database, DatabaseEntry
>>> database = Database('sqlite:///:memory:')
>>> database.create_tables()
>>> entry = DatabaseEntry()
>>> database.tags
[]
>>> entry.tags
[]
>>> database.tag(entry, 'shiny', 'bright', 'yellow')
>>> entry.tags
[<Tag(id None, name 'bright')>, <Tag(id None, name 'shiny')>, <Tag(id None, name 'yellow')>]
>>> database.tags
[]
>>> database.add(entry)
>>> database.tags
[<Tag(id 1, name 'bright')>, <Tag(id 2, name 'shiny')>, <Tag(id 3, name 'yellow')>]
get entries by tag(s)
To get all entries that have certains tags assigned, the Database class has the new method get_by_tags. It receives at least one tag name (that means a string, not an instance of Tag!) and returns a list of DatabaseEntry instances which have at least one of the given tags assigned.
What about removing tags?
Removing tags is not implemented yet. Because there is a many-to-many relationship, this is not trivial. I will have to read Deleting Rows from the Many to Many Table from the SQLAlchemy Documentation to find out what to consider when removing entries in a many-to-many relationship.