I've got a series of documents that have a version field.
{
"_id": "xxx",
"_rev": "6-4bdeb530234c454ae2f16d77ba577428",
"name": "glossary",
"locale": "en-us",
"version": "186.0",
"title": "Glossary",
...
}
I'd like to be able to return the document where name is glossary that has the largest version.
If I make the map function like this:
function(doc) {
emit(doc.name, doc);
}
and do a reduce on it, I can figure out which document has the latest version, but when I try to return it (return latest_doc), I get a reduce_overflow_error error.
It seems like I need to do this via a map function only, but I can't figure out how to return a single document with the highest value from the map.
I'm sure there's an easy way to do this, but I haven't been able to figure it out.
Can you help me get the latest version of my glossary document?
reduce_overflow_error is couchdb's way of telling you that you are not using reduce in a way it is supposed to be used. From the wiki
Reduce is a powerful feature of CouchDB but is often misused which leads to performance problems. From 0.10 onwards, CouchDB uses a heuristic to detect reduce functions that won't scale to give the developer an early warning. A reduce function must reduce the input values to a smaller output value. If you are building a composite return structure in your reduce, or only transforming the values field, rather than summarizing it, you might be misusing this feature
Try this map function
function(doc) {
emit([doc.name,doc.version], doc._id);
}
query it with
/view-name?include_docs=true&startkey=["glossary",{}]&endkey=["glossary"]&descending=true&limit=1
This will give you the the file with the highest version number. Remove the limit parameter if you want more files.
Related
Considering:
doc profile
{
_id:"1",
name:"john",
likes: ["2222","1111"]
}
doc likes
{
_id:"2222",
value:"true"
}
{
_id:"1111",
value:"false"
}
I have a filter on my xamarin app to get the profile, and it works well but I need to include the "children" (linked) docs... I can do this with a view setting include_docs=true but I want couchdb to filter so I can use replication.
Also, it would be possible to accomplish the same result if I could use a reduce function to filter data, but I can't make the filter use the reduce function.. So, any idea?
the expected result would be:
doc profile
{
_id:"1",
name:"john",
likes: {
{_id:"2222",
value:"true"},
{_id:"1111",
value:"false"]
}
}
Thanks!
I can do this with a view setting include_docs=true but I want couchdb to filter so I can use replication
You might already know this but you can use couchdb views as filters.
Also, it would be possible to accomplish the same result if I could use a reduce function to filter data
The reduce function is for "reducing" the values that are returned by the map function. The map function returns a key and a value like so:
emit(key,value)
The reduce function only gets the keys and the values that are returned from a map function. For example if you call a view with
?key=abc
and it returns results like
[{
_id:...,
type: abc
},
{
_id:...,
type:abc
}
....
]
You already have all the documents filtered by the key "abc". The reduce function will get as inputs the key, the value and a rereduce parameters. If you use the reduce function as a post map processing step to further filter the results from the view there will be two problems:
There is no way to pass a parameter to a reduce. The keys that you specify will only be used by the map function and then passed as they are to reduce.
It is not a good idea anyway. With reduce you want to return a small value that aggregates the results you get from a view. So taking the above example if you return say an integer as a value from the map function ( in emit(key,value)//suppose that the value is an integer) the reduce function may return a sum or aggregate of those values. But trying to return a modified document is not what reduce function is for. From the docs
"A reduce function must reduce the input values to a smaller output value. If you are building a composite return structure in your reduce, or only transforming the values field, rather than summarizing it, you might be misusing this feature. "
List functions might be more suited to what you are trying to do. If you want to process the results of the view query before returning them they are they way to go.
In list functions you get a set of results returned by the view function. You can even pass additional parameters if you'd like to apply complex filters on them. But you won't be able to use list functions for replication.
Finally replication works on a document level. Documents have _rev fields that is used by the replicator process to check what version the document is in before the replication is performed. So you won't be able to replicate the results returned by a view. Only the documents will be replicated.
I am trying to find an nth object using '_id', which is in the same document.
Any suggestions or references or code samples would be appreciated.
(e.g)
Document will look as below:
{
"_id": "xxxxx",
"name": "One",
"pocket": [{
"_id": "xxx123",
"name": "NestedOne",
"pocket": []
}, {
"_id": "xxx1234",
"name": "NestedTwo",
"pocket": [{
"_id": "xxx123456",
"name": "NestedTwoNested",
"pocket": [{"_id": "xxx123666",
"name": "NestedNestedOne",
"pocket": []
}]
}]
}]
}
The pockets shall hold more pockets and it is dynamic.
Here, I would like to search "pocket" using "_id" , say "xxx123456", but without using static reference.
Thanks again.
I highly recommend you change your document structure to something easier to manage/search, as this will only become more of a pain to work with.
Why not use multiple collections, like explained in this answer?
So an easy way to think about this for your situation, which I hope is easier for you to reason about than dropping some schema code...
Store all of your things as children in the same document. Give them unique _ids.
Store all of the contents of their pockets as collections. The collections simply hold all the ids that would normally be inside the pocket (instead of referencing the pockets themselves).
That way, a lot of your work can happen outside of DB calls. You just batch pull out the items you need when you need them, instead of searching nested documents!
However, if you can work with the entire document:
Looks like you want to do a recursive search a certain of number of levels deep. I'll give you a general idea with some pseudocode, in hopes that you'll be able to figure the rest out.
Say your function will be:
function SearchNDeep(obj, n, id){
/**
You want to go down 1 level, to pocket
see if pocket has any things in it. If so:
Check all of the things...for further pockets...
Once you've checked one level of things, increment the counter.
When the counter reaches the right level, you'd want to then see if the object you're checking has a `'_id'` of `id`.
**/
}
That's the general idea. There is a cleaner, recursive way to do this where you call SearchNDeep while passing a number for how deep you are, base case being no more levels to go, or the object is found.
Remember to return false or undefined if you don't find it, and the right object if you do! Good luck!
I am scraping an 90K record database using JSON-RPC and I am trying to put in some basic error checking. I want to start by scraping the database twice using two different settings and adding a prefix to the second scrape. This way I can check to ensure that the two settings are not producing different records (due to dropped updates, etc). I wanted to implement the comparison using a view which compares each document from the first scrape with it's twin produced by the second scrape and then emit the names of records with a difference between them.
However, I cannot quite figure out how to pull in another doc in the view, everything I have read only discusses external docs using the emit() function, which is too late to permit me to compare it. In the example below, the lookup() function would grab the referenced document.
Is this just not possible?
function(doc) {
if(doc._id.slice(0,1)!=='$' && doc._id.slice(0,1)!== "_"){
var otherDoc = lookup('$test" + doc._id);
if(otherDoc){
var keys = doc.value.keys();
var same = true;
keys.forEach(function(key) {
if ((key.slice(0,1) !== '_') && (key.slice(0,1) !=='$') && (key!=='expires')) {
if (!Object.equal(otherDoc[key], doc[key])) {
same = false;
}
}
});
if(!same){
emit(doc._id, 1);
}
}
}
}
Context
You are correct that this is not possible in CouchDB. The whole point of the map function is that it must be idempotent, otherwise you lose all the other nice benefits of a pre-calculated index.
This is why you cannot access external resources in the map function, whether they be other records or the clock. Any time you run a map you must always get the same result if you put the same record into it. Since there are no relationships between records in CouchDB, you cannot promise that this is possible.
Solution
However, you can still achieve your end goal, just be different means. Some possibilities...
Assuming there is some meaningful numeric value in each doc, you could use a view to take the sum of all those values and group them by which import you did ({key: <batch id>, value: <meaningful number>}). Then compare the two numbers in your client or the browser to see if they match.
A brute force approach would be to use a view to pair the docs that should match. Each doc is on a different row, but they're grouped by a common field. Then iterate through the entire index comparing the pairs. This would certainly be the quickest to code and doesn't depend on your application or data.
Implement a validation function to enforce a schema on your data. Just be warned that this will reduce your write throughput since each written record will be piped out of Erlang and into the JS engine. Also, this is only applicable if you're worried about properly formed records instead of their precise content, which might not be the case.
Instead of your different batch jobs creating different docs, have them place them into the same doc. The structure might look like this: { "_id": "something meaningful", "batch_one": { ..data.. }, "batch_two": { ..data.. } } Then your validation function could compare them or you could create a view that indexes all the docs that don't match. All depends on where in your pipeline you want to do the error checking and correction.
Personally I like the last option better, but only if you don't plan to use the database as is in production. Ie., you wouldn't want to carry around all that extra data in each record.
Hope that helps.
Cheers.
I'm tryting to decide the best approach for a CouchApp (no middleware). Since there are similarities to my idea, lets assume we have a stackoverflow page stored in a CouchDB. In essence it consists of the actual question on top, answers and commets. Those are basically three layers.
There are two ways of storing it. Either within a single document containing a suitable JSON representation of the data, or store each part of the entry within a separate document combining them later through a view (similar to this: http://www.cmlenz.net/archives/2007/10/couchdb-joins)
Now, both approaches may be fine, yet both have massive downsides from my current point of view. Storing a busy document (many changes through multiple users are expected) as a signle entity would cause conflicts to happen. If user A stores his/her changes to the document, user B would receive a conflict error once he/she is finished typing his/her update. I can imagine its possible to fix this without the users knowledge through re-downloading the document before retrying.
But what if the document is rather big? I'll except them to become rather blown up over time which would put quite some noticeable delay on a save process, especially if the retry process has to happen multiple times due to many users updating a document at the same time.
Another problem I'd see is editing. Every user should be allowed to edit his/her contributions. Now, if they're stored within one document it might be hard to write a solid auth handler.
Ok, now lets look at the multiple documents approach. Question, Answers and Comments would be stored within their own documents. Advantage: only the actual owner of the document can cause conflicts, something that won't happen too often. Being rather small elements of the whole, redownloading wouldn't take much time. Furthermore the auth routine should be quite easy to realize.
Now here's the downside. The single document is real easy to query and display. Having a lot of unsorted snippets laying around seems like a messy thing since I didn't really get the actual view to present me with a 100% ready to use JSON object containing the entire item in an ordered and structured format.
I hope I've been able to communicate the actual problem. I try to decide which solution would be more suitable for me, which problems easier to overcome. I imagine the first solution to be the prettier one in terms of storage and querying, yet the second one the more practical one solvable through better key management within the view (I'm not entirely into the principle of keys yet).
Thank you very much for your help in advance :)
Go with your second option. It's much easier than having to deal with the conflicts. Here are some example docs how I might structure the data:
{
_id: 12345,
type: 'question',
slug: 'couchdb-single-document-vs-joining-documents-together',
markdown: 'Im tryting to decide the best approach for a CouchApp (no middleware). Since there are similarities to...' ,
user: 'roman-geber',
date: 1322150148041,
'jquery.couch.attachPrevRev' : true
}
{
_id: 23456,
type: 'answer'
question: 12345,
markdown: 'Go with your second option...',
user : 'ryan-ramage',
votes: 100,
date: 1322151148041,
'jquery.couch.attachPrevRev' : true
}
{
_id: 45678,
type: 'comment'
question: 12345,
answer: 23456,
markdown : 'I really like what you have said, but...' ,
user: 'somedude',
date: 1322151158041,
'jquery.couch.attachPrevRev' : true
}
To store revisions of each one, I would store the old versions as attachments on the doc being edited. If you use the jquery client for couchdb, you get it for free by adding the jquery.couch.attachPrevRev = true. See Versioning docs in CouchDB by jchris
Create a view like this
fullQuestion : {
map : function(doc) {
if (doc.type == 'question') emit([doc._id, null, null], null);
if (doc.type == 'answer') emit([doc.question, doc._id, null], null);
if (doc.type == 'comment') emit([doc.question, doc.answer, doc._id], null) ;
}
}
And query the view like this
http://localhost:5984/so/_design/app/_view/fullQuestion?startkey=['12345']&endkey=['12345',{},{}]&include_docs=true
(Note: I have not url encoded this query, but it is more readable)
This will get you all of the related documents for the question that you will need to build the page. The only thing is that they will not be sorted by date. You can sort them on the client side (in javascript).
EDIT: Here is an alternative option for the view and query
Based on your domain, you know some facts. You know an answer cant exist before a question existed, and a comment on an answer cant exist before an answer existed. So lets make a view that might make it faster to create the display page, respecting the order of things:
fullQuestion : {
map : function(doc) {
if (doc.type == 'question') emit([doc._id, doc.date], null);
if (doc.type == 'answer') emit([doc.question, doc.date], null);
if (doc.type == 'comment') emit([doc.question, doc.date], null);
}
}
This will keep all the related docs together, and keep them ordered by date. Here is a sample query
http://localhost:5984/so/_design/app/_view/fullQuestion?startkey=['12345']&endkey=['12345',{}]&include_docs=true
This will get back all the docs you will need, ordered from oldest to newest. You can now zip through the results, knowing that the parent objects will be before the child ones, like this:
function addAnswer(doc) {
$('.answers').append(answerTemplate(doc));
}
function addCommentToAnswer(doc) {
$('#' + doc.answer).append(commentTemplate(doc));
}
$.each(results.rows, function(i, row) {
if (row.doc.type == 'question') displyQuestionInfo(row.doc);
if (row.doc.type == 'answer') addAnswer(row.doc);
if (row.doc.type == 'comment') addCommentToAnswer(row.doc)
})
So then you dont have to perform any client side sorting.
Hope this helps.
I tried adding my own fields with names like _myappvar and _myotherappvar to documents to distinguish them from data fields. At first it worked but at some point futon starts to complain.
What is the right way to go?
I am using couchdb 0.9.0, this may be old, butI will not be able to upgrade in this iteration.
Edit: I guess _* is reserved for couchdb vars. I could choose something else but is there a best practice or ho are you solving this?
Edit2: This is somehow severe for my application, because it is already live with those fields. I wonder under which circumstances I can keep the parts that work and only apply a new naming for future fields.
You are correct. The CouchDB Document API, Special Fields section explains it.
Top-level fields may not begin with _.
CouchDB is relaxed, so the best way to go is the easiest thing for your application. About your specific edits:
One idea is to use the _ suffix instead of a prefix. Another idea is a .myapp field which is an object(namespace) for your internal data. You could combine them too:
{
"type": "the document type",
"var1": "Normal variable 1",
"var2": true,
"myapp_": {
"var": "Something internal",
"othervar": null,
}
}
Now you can reference doc.myapp_.var in your view maps, reduces, etc.
You have a choice. You can bite the bullet and change all documents right now. I don't know your app however I prefer that because you are playing with fire using a _ prefix.
However, you could also have both types of document and simply teach your map() function how to handle both of them.
function(doc) {
if(doc.type == "the document type") {
if(doc._myappvar) {
emit(doc._id, doc._myappvar); // The old way
} else if(doc.myapp_) {
emit(doc._id, doc.myapp_.var); // The new way
}
}
}
Good luck!