How to query array columns in Rails 4?

1. Usually, associations are a preferable way of approaching the problem:

   Book has_many :subjects # or has_one/has_and_belongs_to_many

   Subject belongs_to :book # or has_and_belongs_to_many

   And then just create a table subjects, save all your subjects there and you're set up.

2. Your queries:

Query books that contains any of the listed subjects - e.g. Fiction OR Biography

Book.find_by_sql "SELECT * FROM books WHERE 'Fiction' = ANY (subjects) OR 'Biography' = ANY (subjects)"

Query books that doesn't contain a certain subject - e.g. NOT Physics

Book.where.not("subjects @> ?", "{Physics}")

Query books that doesn't contain ANY of the subjects - e.g. NOT (Physics OR Chemistry OR Biology)

Book.find_by_sql "SELECT * FROM books WHERE books NOT IN (SELECT * FROM books WHERE 'Physics' = ANY (subjects) OR 'Chemistry' = ANY (subjects) OR 'Biology' = ANY (subjects)"

For,

Query books that contains any of the listed subjects - e.g. Fiction OR Biography

Book.where("subjects &&  ?", "{Fiction,Biography}")

Query books that doesn't contain a certain subject - e.g. NOT Physics

Book.where("subjects <>  ?", "{Physics}")

Query books that don't contain ANY of the subjects - e.g. NOT (Physics OR Chemistry OR Biology)

Book.where.not("subjects &&  ?", "{Physics,Chemistry,Biology}")

You can see the array functions of Postgres for reference.

https://www.postgresql.org/docs/8.2/functions-array.html