Multiple has_many relationships to same model

How do you get around this problem?

By giving your associations unique names. It's not that you can't unambiguously access them, it's that your second one is destroying the first one.

Instead of calling both posts, use bookmarked_posts for your second association and use source: to call posts

has_many :bookmarked_posts, through: :bookmarks, source: :post

I did something similar to previous comments here but successfully used inverse_of to differentiate users and 'post authors':

User
has_many :authored_posts, class_name: 'Post', inverse_of: 'author'

Maybe something like this?

User
  has_many :bookmarks
  has_many :posts, through: :bookmarks
  has_many :authored_posts, foreign_key: :author_id, class_name: 'Post'
Bookmark
  belongs_to :post
  belongs_to :user
Post
  belongs_to :author, class_name: 'User'
  has_many :bookmarks
  has_many :users, through: :bookmarks

In this way, you are able to keep posts that are written by and bookmarked by a user separate. You could also set it up so that whenever an author creates a post, it can automatically get bookmarked by the user. i.e.

class PostsController < ActionController::Base
  def create
    @post = @user.authored_posts.build(post_params)
    @user.posts << @post

    if @post.valid?
      # do good stuff
    else
      # do errors
    end
  end
end

There is a strong 1:N relationship between author and authored_posts. Then there is a weaker N:M relationship between Users and Posts using Bookmark as the join model. You can add authored posts to be bookmarked when they are created using the controller code above. Removing a bookmark on an authored post will simply remove it from the posts relationship, but not the authored_posts relationship.

You cannot define multiple relationships using the same name.