Cause: The “ActionController::UnknownFormat” error occured because you set HTML as the only format to react to. As a result, you missed a format in a response that you wish to support.

Solution: When writing a blog post, you should consider an action where you want to reply to HTML and JSON queries to help your page be able to support an Ajax request. This is what it may look like:

class BlogPostsController < ApplicationController
def create
@blog_post = BlogPost.new(blog_post_params)
respond_to do |format|
if @blog_post.save
format.html { redirect blog_post_path(@blog_post) }
format.json { render json: @blog_post.to_json }
else
render :new
end
end
end
end

When the blog post fails validations and is not saved, this error is raised. within the response block’s scope of word formatting blocks, You must use the render command. If you were to rewrite this to account for failure, it would look like this:

class BlogPostsController < ApplicationController
def create
@blog_post = BlogPost.new(blog_post_params)
respond_to do |format|
if @blog_post.save
format.html { redirect blog_post_path(@blog_post) }
format.json { render json: @blog_post.to_json }
else
format.html { render :new }
format.json { render json: @blog_post.errors.to_json }
end
end
end
end

And now, there will be no more unintended ActionController::UnknownFormat exceptions that all formats have been declared.