I am getting a ' undefined method error ' when saving a comment

hi!! I am a newbie in ROR.

I have two models

Post
has_many :Comment

Comment
belongs_to :Post

the error displays when I try to save a comment related to certain post.
I get the following error

You shold use

has_many :comments
instead of
has_many :comment

Then you can call

@post.comments

Look this example:
http://guides.rubyonrails.org/association_basics.html#the-has-many-through-association

Take a look at accepts_nested_attributes to create your posts:
http://api.rubyonrails.org/classes/ActiveRecord/NestedAttributes/ClassMethods.html

hi!! I am a newbie in ROR.

I have two models

Post
has_many :Comment

that should be
has_many :comments
note that it is plural and lower case

Comment
belongs_to :Post

belongs_to :post

the error displays when I try to save a comment related to certain post.
I get the following error
-----------------------------------------------------------------
undefined method `comment' for nil:NilClass
-----------------------------------------------------------------
my comment controllers

  def new
@comment = Comment.new
render :layout => false
  end

  def create
    @post = Post.find_by_id(params[:id])
    @post_comment = @post.comment.create(params[:comment])

You have not told us which line the error points to but I guess it is
the one above. If so then it is saying that @post is nil, so your
find_by_id did not find anything. You don't need to use find_by_id,
you can just use
@post = Post.find( params[:id] )
Perhaps there is not a Post object with the correct id. If you look
in log/development.log you will see the params you are passing which
may give you a clue.

I suggest, however, that as a beginner you work right through a good
tutorial such as railstutorial.org (which is free to use online).
That will show you the basics of rails.

Also see the Rails Guides.

Colin