In App-Engine DataStore (which while based on BigTable isn't actully
'BigTable')
You have StringListProperties
https://developers.google.com/appengine/docs/python/datastore/typesandpropertyclasses#StringListProperty
So can just store the tag(s) in a string list property on your CONTENT
model. No TC or TAG model!
You can actully filter string lists, where an equality query actually says
where the list contains this value. (case 2)
Can then do queries that look for entities, that contain this value AND
this value (3)
Can also combine one 'range' filter with such queries to do your (case 4)
Luckily, you can also mostly avoid a historic problem with multi-list
queries
https://developers.google.com/appengine/articles/indexselection#Exploding_Search
Basically you 'Normalize' the database. With a relational database you are
used to denormalizing, don't do it with DataStore.
With the datastore like this, you can't easily get a list of all tags,
with your RDMS could just query the TAG table. If need a list of tags,
store that seperately.
---
Also your TC.tag_key.IN, mentioned in the question. Looks for entities,
with EITHER of the tags mentioned in the query. Not both.
https://developers.google.com/appengine/docs/python/ndb/queries#repeated_properties
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