I also want to get the custom columns via REST, If you fire up Google Postman or Advanced REST Api client you can get a desired json response for the query below.
https://mydev/sites/site1/_api/SP.AppContextSite(@target)/web/lists/getbytitle('Posts')/items?@target='https://mydev/sites/site2/subsiteBlog'
But in the Post's list I made a custom column and the custom field doesn't come with that response, How can I get the value from this field using a rest api call ?