Select DISTINCT individual columns in django

Question

I m curious if there s any way to do a query in Django that s not a  SELECT   FROM     underneath  I m trying to do a  SELECT DISTINCT columnName FROM      instead   Specifically I have a model that looks like   class ProductOrder models Model      Product    models CharField max length 20  promary key True     Category   models CharField max length 30     Rank   models IntegerField       where the Rank is a rank within a Category  I d like to be able to iterate over all the Categories doing some operation on each rank within that category   I d like to first get a list of all the categories in the system and then query for all products in that category and repeat until every category is processed   I d rather avoid raw SQL  but if I have to go there  that d be fine  Though I ve never coded raw SQL in Django Python before

User · Answer

It s quite simple actually if you re using PostgreSQL  just use distinct columns   documentation    Productorder objects all   distinct  category     Note that this feature has been included in Django since 1 4

User · Answer

One way to get the list of distinct column names from the database is to use distinct    in conjunction with values     In your case you can do the following to get the names of distinct categories   q   ProductOrder objects values  Category   distinct   print q query   See for yourself     The query would look something like   SELECT DISTINCT  app productorder   category  FROM  app productorder    There are a couple of things to remember here  First  this will return a ValuesQuerySet which behaves differently from a QuerySet  When you access say  the first element of q  above  you ll get a dictionary  NOT an instance of ProductOrder    Second  it would be a good idea to read the warning note in the docs about using distinct    The above example will work but all combinations of distinct   and values   may not    PS  it is a good idea to use lower case names for fields in a model  In your case this would mean rewriting your model as shown below   class ProductOrder models Model       product    models CharField max length 20  primary key True      category   models CharField max length 30      rank   models IntegerField

User · Answer

User order by with that field  and then do distinct   ProductOrder objects order by  category   values list  category   flat True  distinct

User · Answer

The other answers are fine  but this is a little cleaner  in that it only gives the values like you would get from a DISTINCT query  without any cruft from Django    gt  gt  gt  set ProductOrder objects values list  category   flat True    u category1   u category2   u category3   u category4     or   gt  gt  gt  list set ProductOrder objects values list  category   flat True     u category1   u category2   u category3   u category4     And  it works without PostgreSQL   This is less efficient than using a  distinct    presuming that DISTINCT in your database is faster than a python set  but it s great for noodling around the shell

[django] Select DISTINCT individual columns in django?

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