Query on Postgres JSON array field in Rails

Assumptions:

• Postgres 9.4 or later.
• "get all the users that are in serie 5" is supposed to mean:
  "with at least one array element that contains {"serie": 5}. There may be others."
• Working with your first, shorter data format. No redundant 'data' key.

Short answer: Use jsonb instead of json and this just works:

User.where("groups @> ?", '[{"serie": 5}]')

Note the square brackets to make the right-hand operand a JSON array.

Why?

The prominent misunderstanding here: data type json is not the same as jsonb.

You didn't declare the actual table definition, but you later commented json and there is a hint in the question:

select json_array_elements(groups -> 'data') ->> 'serie' from users;

json_array_elements() only works for json, would have to be jsonb_array_elements() for jsonb.
But you try to use the jsonb Operators @>, which is impossible for json:

groups -> 'data' @>  '?'

The operator -> returns the same type as the left-hand input. But @> is only defined for jsonb, not for json.

Then you try to use the operator @> for text as left-hand operand. Not possible either:

groups ->> 'data' @>  ?

There are variants of the operator @> for various types (incl. Postgres arrays), but not for text and not for json.

So, the short answer: Use jsonb instead of json. This allows to use very efficient indexes, too:

• Index for finding an element in a JSON array

json

For data type json you could use:

SELECT *
FROM   users u
WHERE  EXISTS (
   SELECT 1
   FROM   json_array_elements(u.groups) elem 
   WHERE  elem ->> 'serie' = '5'
   );

Demos

jsonb:

SELECT *
FROM  (
   VALUES (1, jsonb '[{"serie":5, "year":3, "specialization":"Matematica", "management_id":1, "group_number":2}
                    , {"serie":5, "year":3, "specialization":"Matematica", "management_id":1, "group_number":2}]')
        , (2,       '[{"serie":7, "year":3, "specialization":"Matematica", "management_id":1, "group_number":2}
                    , {"serie":8, "year":3, "specialization":"Matematica", "management_id":1, "group_number":2}]')
        , (3,       '[{"serie":9, "year":3, "specialization":"Matematica", "management_id":1, "group_number":2}
                    , {"serie":5, "year":3, "specialization":"Matematica", "management_id":1, "group_number":2}]')
   ) users(id, groups)
WHERE  groups @> '[{"serie": 5}]';

json:

SELECT *
FROM  (
   VALUES (1, json  '[{"serie":5, "year":3, "specialization":"Matematica", "management_id":1, "group_number":2}
                    , {"serie":5, "year":3, "specialization":"Matematica", "management_id":1, "group_number":2}]')
        , (2,       '[{"serie":7, "year":3, "specialization":"Matematica", "management_id":1, "group_number":2}
                    , {"serie":8, "year":3, "specialization":"Matematica", "management_id":1, "group_number":2}]')
        , (3,       '[{"serie":9, "year":3, "specialization":"Matematica", "management_id":1, "group_number":2}
                    , {"serie":5, "year":3, "specialization":"Matematica", "management_id":1, "group_number":2}]')
   ) users(id, groups)
WHERE  EXISTS (
   SELECT 1
   FROM   json_array_elements(users.groups) elem 
   WHERE  elem ->> 'serie'  = '5'
   );