I have a game grid that I'm trying to fit into an array of rows each with a number of blocks. The blocks are stored in the same collection and have row and column fields.
My current attempt is as follows.
exports.findById = function(req,res) {
var id = req.params.id;
console.log('Retrieving game #' + id);
db.games.findOne( { "_id" : ObjectId(id) }, function(err, game_obj) {
if(err || !game_obj) {
console.log("no game");
res.send({'error':'An error has occurred'});
} else {
var rows = [];
function getRowBlocks(err,blocks) {
if(err || !blocks) {
console.log("no blocks");
res.send({'error':'An error has occurred'});
} else {
console.log(blocks[0].row + " has " + blocks.length + " blocks");
rows[blocks[0].row] = blocks;
}
}
for(i=0;i<game_obj.farthest_row;i++) {
db.blocks.find( { "game_id" : ObjectId(id), "row" : i }).sort( { "column" : 1 }).toArray(getRowBlocks);
}
res.send('game', { row_list : rows, game : game_obj});
}
});
}
But due to the scope of the mongodb find operation called for each row I cannot actually get the blocks stored in the rows variable. And because there are multiple operations run I cannot simple put the res.send... inside the toArray callback.
I could only use one query and build the array inside the callback and then pass that on, but I think this would be a lot less efficient considering that the multiple queries are happening asynchronously and that there could be a large number of rows.
I think I could do it if I could pass the rows array into the query callback, but I haven't been able to find out if that's possible.
That's what async.parallel is for:
// build a list of query-functions
var queries = [];
for (var i = 0; i < game_obj.farthest_row ; i++) {
queries.push(function(i, cb) {
db.blocks
.find( { "game_id" : ObjectId(id), "row" : i })
.sort( { "column" : 1 })
.toArray(cb);
}.bind(this, i)); // create a partial function to create a newly scoped 'i'
}
// pass the query functions to async, which will run them in parallel,
// gather all the results, and finally call the callback function with
// the results-array
async.parallel(queries, function(err, results) {
if (err)
// handle error
// results now contains the query results (array-in-array)
results.forEach(function(blocks) {
getRowBlocks(null, blocks); // this probably requires a rewrite...
});
});