I'm using CouchDB for storing data about events. Each event has a start date/time and end date/time. I'd like to create a view now, that allows me to get a list of all events that happen on a specific date/time. But the events don't have a single dates, they can rather range over several days.
Now I don't know how I can reflect this in my view function. Unfortunately, I need granularity on minute level, so emitting a key for each minute might not be a valid solution. So how can I do that?
Thanks in advance!
Ok, here's a solution anyway! I just ping-ponged with Jan (Lehnardt) of CouchDB and he told me that I can emit() multiple times within a map. Something I did not know up until now.
To make it easier for myself, I'm assuming your end and start time are TIMESTAMP values already. If not, you need to convert them in your map or generally switch to them.
I'm also gonna assume that an event starts at a full minute (e.g. 16:51:00) and not at 16:51:23. Same on the end date.
Example document:
{
"_id" : "super-random-id",
"name" : "event 1",
"start" : "TIMESTAMP",
"end" : "TIMESTAMP"
}
Here's the map:
function (doc) {
if (doc.start == undefined || doc.end == undefined) {
return;
}
var current = doc.start;
while (current <= doc.end) {
emit(current, doc.id);
current = current + 60; // increment by 1 minute
}
}
Then it should be easy to query with startkey and endkey. You could probably add an _list here.
I found finally a good solution using GeoCouch: http://www.diretto.org/2010/08/efficient-time-based-range-queries-in-couchdb-using-geocouch/
Related
So I got a small database, It's not going to grow much more and I'm trying to get one document from the db in an API that I implemented in python so that with a given document Id I retrieve the document in the db. However, I find it a little hard to put the user to write a random number from the db. All I require is a function that modifies each document by setting an id field and to Auto-Increment. As I said, it's not going to grow that much and the performance isn't really an issue here.
So far what I've been able to do is this:
var i = 0
db.MyCollection.update({},
{$set : {"new_field":1}},
{upsert:false,
multi:true}
i ++;),
I achieved to set an id field but it sets the same number to each document (the count of every document) So let's say that if the db has 10 docs, it'll set the Id to 10.
Find-and-modify operation returns the document updated (before or after the update depending on returnDocument setting). You can use this with $inc to implement a counter. Ruby example where c is a collection:
irb(main):005:0> c['foo'].insert_one(counter:true,count:1)
=> #<Mongo::Operation::Insert::Result:0x8040 documents=[{"n"=>1, "opTime"=>{"ts"=>#<BSON::Timestamp:0x00005609f260b7e0 #seconds=1594961771, #increment=2>, "t"=>1}, "electionId"=>BSON::ObjectId('7fffffff0000000000000001'), "ok"=>1.0, "$clusterTime"=>{"clusterTime"=>#<BSON::Timestamp:0x00005609f260b538 #seconds=1594961771, #increment=2>, "signature"=>{"hash"=><BSON::Binary:0x8060 type=generic data=0x0000000000000000...>, "keyId"=>0}}, "operationTime"=>#<BSON::Timestamp:0x00005609f260b290 #seconds=1594961771, #increment=2>}]>
irb(main):011:0> c['foo'].find_one_and_update({counter:true},{'$inc':{count:1}})
=> {"_id"=>BSON::ObjectId('5f112f6b2c97a6281f63f575'), "counter"=>true, "count"=>1}
irb(main):012:0> c['foo'].find_one_and_update({counter:true},{'$inc':{count:1}})
=> {"_id"=>BSON::ObjectId('5f112f6b2c97a6281f63f575'), "counter"=>true, "count"=>2}
irb(main):013:0> c['foo'].find_one_and_update({counter:true},{'$inc':{count:1}})
=> {"_id"=>BSON::ObjectId('5f112f6b2c97a6281f63f575'), "counter"=>true, "count"=>3}
irb(main):014:0> c['foo'].find_one_and_update({counter:true},{'$inc':{count:1}})
=> {"_id"=>BSON::ObjectId('5f112f6b2c97a6281f63f575'), "counter"=>true, "count"=>4}
Why not just use this logic? Instead of updating all via one query, just launch multiple queries one by one? Mongo will do it pretty fast, even if you have >1M docs in database (according to your phrase: I got a small database) because pre-builded index on _id field.
this is a javasript code, but I guess, you'll understand the logic of it
let all_documents = db.MyCollection.find({});
for (let i = 0; i < all_documents.length; i++) {
db.MyCollection.update({_id: all_documents[i]._id }, {$set : {"new_field": i}}, {upsert:false})
}
I'm using a MongoDB mapReduce to code a ranking feed algorithm, it almost works but the latest thing to implement is the pagination. The map reduce supports the results limitation but how could I implement the offset (skipping) based e.g. on the latest viewed _id of the results, knowing that I'm using mongoose?
This is the procedure I wrote:
o = {};
o.map = function() {
//log10(likes+comments) / elapsed hours from the post creation
emit(Math.log(this.likes + this.comments + 1) / Math.LN10 / Math.abs((now - this.createdAt) / 6e7 + 1), this);
};
o.reduce = function(key, values) {
//sort the values, when they have the same score
values.sort(function(a, b) {
a.createdAt - b.createdAt;
});
//serialize the values, because mongoose does not support multiple returned values
return JSON.stringify(values);
};
o.scope = {now: new Date()};
o.limit = 15;
Posts.mapReduce(o, function(err, results) {
if (err) return console.log(err);
console.log(results);
});
Also, if the mapReduce it's not the way to go, do you suggest other on how to implement something like this?
What you need is a page delimiter which is not the id of the latest viewed as you say, but your sorting property. In this case, it seems to be the formula Math.log(this.likes + this.comments + 1) / Math.LN10 / Math.abs((now - this.createdAt) / 6e7 + 1).
So, in your mapReduce query needs to hold a where value of that formula above. Or specifically, 'formula >= . And also it needs to hold the value of createdAt at the last page, since you don't sort by that. (Assuming createdAt is unique). So yourqueryof mapReduce would saywhere: theFormulaExpression, createdAt: { $lt: lastCreatedAt }`
If you do allow multiple identical createdAt values, you have to play a little outside of the database itself.
So you just search by formula.
Ideally, that gives you one element with exactly that value, and the next ones sorted after that. So in reply to the module caller, remove this first element off the array (and make sure you actually ask for more results then you need because of this).
Now, since you allow for multiple similar values, you need another identifying prop, say, object id or created_at. Your consumer (caller of this module) will have to provide both (last value of the score, createdAt of the last object). Say you have a page split exactly in the middle - one or more objects is on the previous page, another set on the next
. You'd have to not simply remove the top value (because that same score is already served on the previous page), but possibly several of them from the top.
Then it goes really crazy, because potentially your whole page was already served - compare the _ids, look for the first one after the one your module caller has provided you with. Or look into the data and determine how many matching values like that are there, try to get at least as many more values from mapReduce then you have on your actual page size.
Aside from that, I would do this with aggregation instead, it should be much more preformant.
I want to be able query the couchdb between dates, I know that this can be done with startkey and endkey (it works fine), but is it possible to do query for example like this:
SELECT *
FROM TABLENAME
WHERE
DateTime >= '2011-04-12T00:00:00.000' AND
DateTime <= '2012-05-25T03:53:04.000'
AND
Status = 'Completed'
AND
Job_category = 'Installation'
Generally-speaking, establishing indexes on multiple fields grows in complexity as the number of fields increases.
My main question is: do Status and Job_category need to be queried dynamically too? If not, your view is simple:
function (doc) {
if (doc.Status === 'Completed' && doc.Job_category === 'Installation') {
emit(doc.DateTime); // this line may change depending on how you break up and emit the datetimes
}
}
Views are fairly cheap, (depending on the size of your database) so don't be afraid to establish several that cover different cases. I would expect something like Status to have predefined list of available options, as oppposed to Job_category which seems like it could be more related to user input.
If you need those fields to be dynamic, you can just add them to the index as well:
function (doc) {
emit([ doc.Status, doc.Job_category, doc.DateTime ]);
}
Then you can use an array as your start_key. For example:
start_key=["Completed", "Installation", ...]
tl;dr: use "static" views where you have a predetermined list of values for a given field. while possible to query "dynamic" views with multiple fields, the complexity grows very quickly.
I'm stumbling a bit with my CouchDB knowledge.
I have a database of content that is tagged with an array of tags and has a created date.
I want to create a view that pulls a limited number of newest stories tagged with a specific tag.
For example, the newest 6 stories tagged "Business."
Ran across this question, which seems to get me almost to where I need to go, but I'm missing one key element, which I think is how to craft the query string to sort by one key while searching by the other.
Here's my map function.
function(doc) {
if (doc.published == "yes" && doc.type == "news") {
for (var i = 0; i < doc.tags.length; i++) {
if (doc.tags[i]) {
emit([doc.created, doc.tags[i]], doc);
}
}
}
}
So how do I query that view for a all documents tagged "Business" that are the newest documents based on created.
The created attribute is a date sortable format.
First, I would switch the order of your emit:
emit([doc.tags[i], doc.created]);
(leave out doc as well, you can just add include_docs=true to get the entire document, and your view won't take up so much disk-space in the process)
Now you can query for the all the stories tagged as "Business" by using the following querystring:
startkey=["Business"]&endkey=["Business",{}]
You'll get all the documents with the tag business, and they'll be sorted by date.
This takes advantage of view collation, which basically is the rules governing how indexes are sorted/queried. For complex keys like this, the sorting is done for each item of the array separately. (ie. the first key is sorted first, the second key is sorted second, etc) This is why the order matters, as you must always move from left to right when querying a view index.
If you want the 6 most recent, your querystring will need to change:
descending=true&limit=6&endkey=["Business"]&startkey=["Business",{}]
NOTICE You need to swap the startkey/endkey values, due to how the descending parameter works. See the View reference page on the wiki for further explanation.
OK, I think I figured this out, but I'm not quite certain I fully understand it.
I found this story about complex keys and searching and sorting.
My map function looks like this:
function(doc) {
if (doc.published == "yes" && doc.type == "news") {
for (var i = 0; i < doc.tags.length; i++) {
if (doc.tags[i]) {
emit([doc.tags[i], doc.created], doc);
}
}
}
}
And to query and sort using it, the query looks like this.
http://localhost:5984/database/_design/story/_view/tagged?limit=10&startkey=["Business"]&endkey=["Business",{}]&descending=false
I'm getting the results I want, but I'm not entirely certain I understand it all.
I have records with a time value and need to be able to query them for a span of time and return only records at a given interval.
For example I may need all the records from 12:00 to 1:00 in 10 minute intervals giving me 12:00, 12:10, 12:20, 12:30, ... 12:50, 01:00. The interval needs to be a parameter and it may be any time value. 15 minutes, 47 seconds, 1.4 hours.
I attempted to do this doing some kind of reduce but that is apparently the wrong place to do it.
Here is what I have come up with. Comments are welcome.
Created a view for the time field so I can query a range of times. The view outputs the id and the time.
function(doc) {
emit([doc.rec_id, doc.time], [doc._id, doc.time])
}
Then I created a list function that accepts a param called interval. In the list function I work thru the rows and compare the current rows time to the last accepted time. If the span is greater or equal to the interval I add the row to the output and JSON-ify it.
function(head, req) {
// default to 30000ms or 30 seconds.
var interval = 30000;
// get the interval from the request.
if (req.query.interval) {
interval = req.query.interval;
}
// setup
var row;
var rows = [];
var lastTime = 0;
// go thru the results...
while (row = getRow()) {
// if the time from view is more than the interval
// from our last time then add it.
if (row.value[1] - lastTime > interval) {
lastTime = row.value[1];
rows.push(row);
}
}
// JSON-ify!
send(JSON.stringify({'rows' : rows}));
}
So far this is working well. I will test against some large data to see how the performance is. Any comments on how this could be done better or would this be the correct way with couch?
CouchDB is relaxed. If this is working for you, then I'd say stick with it and focus on your next top priority.
One quick optimization is to try not to build up a final answer in the _list function, but rather send() little pieces of the answer as you know them. That way, your function can run on an unlimited result size.
However, as you suspected, you are using a _list function basically to do an ad-hoc query which could be problematic as your database size grows.
I'm not 100% sure what you need, but if you are looking for documents within a time frame, there's a good chance that emit() keys should primarily sort by time. (In your example, the primary (leftmost) sort value is doc.rec_id.)
For a map function:
function(doc) {
var key = doc.time; // Just sort everything by timestamp.
emit(key, [doc._id, doc.time]);
}
That will build a map of all documents, ordered by the time timestamp. (I will assume the time value is like JSON.stringify(new Date), i.e. "2011-05-20T00:34:20.847Z".
To find all documents within, a 1-hour interval, just query the map view with ?startkey="2011-05-20T00:00:00.000Z"&endkey="2011-05-20T01:00:00.000Z".
If I understand your "interval" criteria correctly, then if you need 10-minute intervals, then if you had 00:00, 00:15, 00:30, 00:45, 00:50, then only 00:00, 00:30, 00:50 should be in the final result. Therefore, you are filtering the normal couch output to cut out unwanted results. That is a perfect job for a _list function. Simply use req.query.interval and only send() the rows that match the interval.