database trouble - same values in different rows - php

Question

I am trying to make my voting system work. I have 3 databse tables:
users, posts, votes
the table users has field username as the primary key. table post has post_id as the primary key. (there are more fields but they don't affect the question/problem)
In the votes table I have 3 fields: username, post_id, vote. vote is enum ('positive', 'negative'). What I'm trying to achieve is that if a user votes for a specific post that is displayed on a page, the query: INSERT INTO votes ('username','post_id','vote') VALUES('$user_name','$post_id', 'positive'); will be executed.
It works if lets say user 123123 has not voted for any post at all yet. When this user votes lets say for post 1, this query works fine. But then if this user wants to vote for a different post, (his vote gets counted - I just copied the part of the code that doesn't work, the rest of it is fine and working) the insert query get's not executed. If user abcd wants to vote for a specific post, this works fine again, but only once. It seems to me that there is some kind of problem with the database, so that there can be only one entry with the same username or post_id. How can I fix this if I want one user to be able to vote for multiple posts? Is there a better strategy for this?
if($runloggedin->num_rows == 1)
{
// If there was no vote for the current posting, then execute this query
$query = "SELECT * FROM posts WHERE post_id='".$post_id."' AND user_name='".$user_name."'"; //get username and the post id
$result = $mysqli->query($query);
$query1 = "SELECT * FROM votes WHERE post_id='".$post_id."' AND username='".$user_name."'"; //check if there is a vote for this post already
$result1 = $mysqli->query($query1);
if ($result->num_rows == 1 && $result1->num_rows == 0)
{
$vote = "INSERT INTO votes ('username','post_id','vote') VALUES('$user_name','$post_id', 'positive')"; // this isn't working. everything else seems to be working (still test it more)
$savevote = $mysqli->query($vote);
$addvote = "UPDATE posts SET posvotes=posvotes+1 WHERE post_id='".$post_id."'";
$runvote = $mysqli->query($addvote);
echo "Thank you for your vote";
}
}

Answer 1

Without seeing how your votes table was created, my guess is that username has been set up as the primary key. This will make the first INSERT work, but all future ones fail. What you need to do is change it to have username & post_id be the primary key
ALTER TABLE `votes` DROP PRIMARY KEY , ADD PRIMARY KEY ( `username`, `post_id` )