Although there can be multiple ways you can get data from your database, but here are two ways you can get data.

1. Using Rails Model -> In this method of data fetching we use rails model, which we create using rails g model model_name.

So the code is as follows:

def index
    @data = ModelName.all
    render_success_response @data
  end

In above piece of code

index -> name of the method

@data -> name of variable to store the data

.all -> rails method to get all the data from the respective table associated with the model

render_success_response -> default rails method to render a success response

Now there can be multiple methods we can use. Check out devhints for references.

1. Using SQL Query -> In this method of data fetching we use direct SQL queries.

So the code is as follows:

def index
    sql = "select * from table_name"
    @data = ActiveRecord::Base.connection.execute(sql)
    render_success_response @data
  end

In above piece of code

ActiveRecord::Base.connection.execute() -> it is used to run sql queries in ruby on rails.

Conclusion
You can use both rails model or sql query in you code according to your use case.

References
Dev Hints