Limit join to one row using knex - greatest-n-per-group

Question

I am trying to fetch data from two tables using a join. The problem is forum_posts will contain several items with the same thread_id. I would only like to get the first one, either by id or by creating date.
function getByGroup(groupId) {
return knex('forum_threads')
.select('forum_threads.id', 'forum_threads.updated_at', 'forum_posts.content')
.where('forum_threads.group_id', '=', groupId)
.leftJoin('forum_posts', function() {
this.on('forum_posts.thread_id', '=', 'forum_threads.id');
})
.orderBy('updated_at', 'desc')
.then(function(threads) {
return threads;
});
}
I would like to add a limit(1) or a min to the join but not entirely sure how to do it.

Answer 1

You need to add a filter like the following to your left join criteria:
.andOn('forum_posts.created_at', '=', knex.raw("(select min(created_at) from forum_posts where forum_posts.thread_id = forum_threads.id)"))
This says to include the forum post record (as a left join match) if it has the minimum updated_at value for that id.
The full code. The below code isn't tested, although I did test the above snippet in a piece of my code.
function getByGroup(groupId) {
return knex('forum_threads')
.select('forum_threads.id', 'forum_threads.updated_at', 'forum_posts.content')
.where('forum_threads.group_id', '=', groupId)
.leftJoin('forum_posts', function() {
this.on('forum_posts.thread_id', '=', 'forum_threads.id')
/* The new line here */
.andOn('forum_posts.created_at', '=', knex.raw("(select min(created_at) from forum_posts where forum_posts.thread_id = forum_threads.id)"))
})
.orderBy('updated_at', 'desc')
.then(function(threads) {
return threads;
});
}
Cheers!
PS: You didn't ask, but something I find very helpful when debugging Knex is the .on() query reporting clauses:
// ...
.orderBy('updated_at', 'desc')
/* your code above */
.on('query', function(data) {
// outputs the SQL query you generated & runtime data bindings.
console.log(data);
})
.on('query-error', function(error, obj) {
// outputs the Knex failed query, data, etc.
console.log("Error:", error);
console.log("Object: ", obj);
})
/* your code below */
.then(function(threads) {
return threads;
});