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Im trying to pull only one column from a datastore table

I have a Books model with id, key, title, author, isbn and price

everything = db.GqlQuery('SELECT * FROM Books') gives me everything, but say i only want the title

books = db.GqlQuery('SELECT title FROM Books')

Ive tried everything people have suggested but nothing seems to work

Any help is much appreciated Thanks

sidarcy
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  • possible duplicate of [Selecting values from a column with GQL in App Engine](http://stackoverflow.com/questions/5681890/selecting-values-from-a-column-with-gql-in-app-engine) – Nick Johnson Aug 29 '11 at 06:51
  • use projection query./. https://cloud.google.com/appengine/docs/python/ndb/projectionqueries – Avinash Raj Sep 12 '16 at 10:40

2 Answers2

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You can't. GQL is not SQL, and the datastore is not a relational database. An entity is stored as a single serialized protocol buffer, and it's impossible to fetch part of an entity; the whole thing needs to be deserialized.

Wooble
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  • Im currently using the free version of app engine. Returning everything is using up a lot of my datastore daily usage so for certain queries id like to be more specific, any suggestions? – sidarcy Sep 07 '11 at 12:10
  • @sidarcy: you could denormalize your models, but IIRC datastore usage is measured in calls to the API, not the size of the returned entities, so being able to return specific columns wouldn't help much with quota. Have you considered caching results in memcache to cut down on calls to the datastore API? – Wooble Sep 07 '11 at 12:33
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A design consideration in the datastore is to use redundancy where you wouldn't in a relational database.

So for example, your code could have a entities for "Titles" that it adds each time a book is added, and removes each time a book is removed. Then you could query the Titles entities to get all the titles without having to load all the books.

Your code has to enforce those rules, but with a bit of python abstraction, that's not difficult (i.e. put all accesses to books or titles behind a class whose methods enforce the relationships in your data).

I highly suggest reading some of the more advanced topics on the data store here.

People often refer to moments of realization as "How I learned to stop worrying and love the data store." They are basically moments when people switch out of a Normal Form way of thinking about data, and into a distributed/redundant view where you can just spray data at the data store and it'll handle it.

vsekhar
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