Document structure:
{
"Type":"post"
"LastModified":"2010-11-01 21:55",
"CategoryID":3,
"ID":12
}
Having a bunch of different post docs in different categories is great. But I can't seem to figure out how to make a view which returns the documents ordered by date, when selecting the ones in, e.g., category 3 and 5. The categories are not known, the limit query should work still.
I've tried different approaches to the view but nothing comes close to achieving the desired result.
In SQL it could probably be done by something like this:
SELECT * FROM document WHERE document.CategoryID in (3,5) ORDER BY document.LastModified DESC;
I could just query a view like this the required number of times, manually sorting and paging the data:
function(doc) {
emit(doc.CategoryID, doc.ID);
}
So does anyone know if it's possible to avoid doing that and just have couchdb be a bit smarter?
I can think of 2 possible solutions.
Emit both CategoryID and LastModified in the map function.
function(doc) {
emit([doc.CategoryID, doc.LastModified], null);
}
Now you can query the view for ?startkey=[3]&endkey=[3, {}] to get all the docs with CategoryID=3 sorted by LastModified. To get docs for multiple CategoryIDs, you need to merge the sorted results.
Use couchdb-lucene to build the index. couchdb-lucene can handle complex queries.
Related
I have a view in couchDb that is defined like this
function (doc) {
if (doc.url) {
var a = new Date(doc.postedOn);
emit([a.toLocaleDateString(), doc.count, doc.userId], {
_id: doc.userId,
postTitle: doc.postTitle,
postSummary: doc.postSummary,
url: doc.url,
count: doc.count
});
}
};
This gives me the result in a format that I want.Sorted first by date then by count and then by userID.
However I have trouble querying it.What I want is to query this view just by userId.That is leave the date and the count parameter null.
_view/viewName?limit=20&descending=true&endkey=["","","userId"]
does not give me the desired result.
Should I be using list function to filter out the results of the view.Is there any impact on performance if I do this?
This quote from the definitive guide first gave me the idea that list functions could be used to filter and aggregate results.
The powerful iterator API allows for flexibility to filter and aggregate rows on the fly, as well as output raw transformations for an easy way to make Atom feeds, HTML lists, CSV files, config files, or even just modified JSON.
List function has nothing to do with your case. From the docs you've linked to yourself:
While Show functions are used to customize document presentation, List functions are used for same purpose, but against View functions results.
Show functions are used to represent documents in various formats, commonly as HTML page with nicer formatting. They can also be used to run server-side functions without requiring a pre-existing document.
To solve your problem just change the order of the emitted keys, putting userId first, i.e.:
[ doc.userId, a.toLocaleDateString(), doc.count ]
and update your query appropriately.
If changing the order of emitted keys is not an option, just create another view.
There's a bunch of questions on here related to pagination using CouchDB, but none that quite fit what I'm wondering about.
Basically, I have a result set ranked by number of votes, and I want to page through the set in descending order.
Here's the map for reference.
function(doc) {
emit(doc.votes);
}
Now, the problem. I found out that startkey_docid doesn't work on it's own. You have to use it in combination with startkey. The thing is, for the query, I don't use a startkey parameter (I'm not looking to restrict the results, just get the most->least). I was thinking I could just use startkey={{doc.votes}}&startkey_docid={{doc._id}} instead, but the number of votes for a document could have changed by the time someone clicks the "Next Page" link.
The way to solve this seemed obvious: just set startkey=99999999 so that it will return all documents in the database and I can just use startkey_docid to start at the one where we left off last time. Oddly, when I do that, the startkey_docid stopped working and just allowed all results to be returned again. Apparently startkey needs to exactly equal the key on the document whose _id is used in startkey_docid.
What I'm asking is whether anyone knows a workaround for using startkey_docid to page when the actual startkey could have changed by the time you want to use it? Should my application just lookup the document by _id and immediately use the doc.votes value hoping it hasn't changed in the few milliseconds between requests? Even that doesn't seem very reliable.
EDIT: Ended up switching to Mongo for the speed, so this question turned out to be kinda moot.
I have never done something like this but I think I have some idea how to do it. What you can do is to take a snapshot of the ratings and refer to it in every page. You probably want your view not to consume to much space, so you should not map separate copies of the documents with votes not changed after taking the snapshot. So, you can do the following:
Add some history of ratings with timestamp to your document.
Map the ratings AND history like this.
In your app get the current time: start_time = Date.now() and query all pages.
Cleanup the history older then the oldest active sessions.
The problem is that if you emit [votes, date] and try to paginate you will never know how many document you have to fetch to get desired number per page. There can always be some older version which you will have to skip, and you will have make next get from DB. Thats why you can consider emitting: [date, votes], read the view always twice -- for start_time and current time, and merge and sort the result (like in merge-sort).
Ad.1:
{ ...,
votes: 12,
history: [
{date: 1357390271342, votes: 10},
{date: 1357390294682, votes: 11}
]
}
Ad.2:
function (doc) {
emit([{}, doc.votes], null);
doc.history && doc.history.forEach(function(h) {
emit([h.date, h.votes], null);
});
}
Ad.3:
?startkey=[start_time, votes]&limit=items_per_page_plus1
?startkey=[{}, votes]&limit=items_per_page_plus1
Merge lists, sort by votes in your app (on in a list function).
If you will have problems with using start_docid then you can emit [date, votes, id] and query with the ID explicitly. Even when this particular doc changes its votes it will still be available in the history.
Ad.4:
If you emit [date, votes] then you can just get outdated history width: ?startkey=[0]&endkey=[oldest_active_session_time]&inclusive_end=false and update them with update handler:
function(doc, req) {
if (!doc || !doc.history) return [null, 'Error'];
var history = new Array();
var oldest = +(req.query.date);
doc.history.forEach(function(h) {
if (h.date >= oldest)
history.push(h);
});
doc.history = history;
return [doc, 'OK'];
}
Note: I have not tested it, so it is expected not to run without modifications :)
As far as I know CouchDB uses b-tree shadowing to make updates and in principle is should be possible to access older revisions of the view. I am not into the CouchDB design, so it is just a guess and there seems not to be any (documented) API for this.
I can't figure out any simple solution by now, but there are options:
Replicate not-so-often your sorting list to small dedicated db so it will be much more stale than stale=ok
Modify your schema in a way that you'll be able to sort by some more stable data. Look at the banking/ledger example in CouchDb guide: http://guide.couchdb.org/draft/recipes.html#banking. Try to log every vote and reduce them hourly for example. As a bonus you'll get a history/trends :)
I'm kind of surprised this question has been left unanswered because the functionality of CouchDB Futon basically does this when you are paginating through the results of a map function. I opened up firebug to see what was happening in the javascript console as I paginated and saw that for every set of paginated results it is passing the startkey along with startkey_docid. So although the question is how do I paginate without including startkey, CouchDB specifies that the startkey is required and demonstrates how it can work. The endkey is not specified, so if there is only one result for the specified startkey, the next set of paginated results will also contain the next key of the sorted results that do not match the startkey.
So to clarify a bit, the answer to this problem is that as you are paginating and keeping track of the startkey_docid, you also need to capture the startkey of the same document that will be the start of the next set of results. When you are calling the paginated results use both the captured startkey and startkey_docid as couchdb requires. Leave endkey off so that the results will continue on to the next key of the sorted results.
The usecase scenario for wanting to be able to paginate without specifying a key is kind of odd. So let's say that the start docid of the next paginated result did change it's key value drastically from a 9 to a 3. And we are also assuming that there is only one instance of the docid existing in the map results, even though it could potentially appear multiple times (which I believe is why the startkey needs to be specified). As the user is clicking the next button, the user's paginated results will have now moved from looking at rank 9 to rank 3. But if you are including the startkey in addition to the startkey_docid, the paginated results would just start all over at the beginning of the rank 9 results which is a more logical progression than potentially jumping over a large set of results.
I prepare to use CouchDB to my project. but cannot find a way to implement a view like an SQL SELECT * FROM Employees WHERE LastName NOT IN (SELECT LastName FROM Managers). In other words, I want to get a set from view A but not in view B. Question: how to implement not-in condition in CouchDB?
Keeping employees and managers lists different sets of documents is using relational structure where you DB is not relational. If, for some reason, you are forced to do that, you need some way to distinguish the scheme of the doc (from which table it is). Lets say you are doing it with field scheme:
{ _id: "EMPL_ID", scheme: "employee", ... }
{ _id: "MNGR_ID", scheme: "manager", employee: "EMPL_ID", ... }
Then you can use map:
function (doc) {
if (!doc.scheme) return;
if (doc.scheme != "manager") emit(doc.last_name, doc);
}
If, for some strange reason, you cannot do that, and you only have the reference to employee doc in manager doc, you can emit both documents:
function (doc) {
if (some_test_for_being_employee_scheme(doc))
emit([doc._id, 1], doc);
if (doc.emp_id)
emit([doc.emp_id, 0], null);
}
You will get the list of employees with keys ["employee_id", 1], and each manager is preceded with the row labeled as manager (key [..., 0]). This will require some space, but with list function you can filter out managers easily and the client will receive from DB only the non-managers.
Keep in mind, that it is only the workaround for not making proper DB design.
If you change the model to make it fit a document-oriented database, this would be easy. I generally keep a "type" key in all of my documents to keep different types of documents straight. If you have a single "person" type and decorate all "person" documents who are also "manager" with a separate key, you could easily emit view keys only for non-managerial personnel. If you opt to have a separate "manager" type, you could similarly restrict emitted view keys to non-managers only.
I think the answer is simply: you can't mix view results. Views are independent.
However, there is a strategy called view collation that probably solves your problems. I suggest reading this: http://wiki.apache.org/couchdb/View_collation
To summarize it: You need to use different document types and then use a single view to collate the results.
I created a simple view to return the blog title and time
function(doc) {
if ( doc.TITLE) emit(doc.TIME, doc.TITLE);
}
what is a simple way to display newest blog articles first (by default it is the other way around)?
Just apply descending sort order at view request e.g.
GET /dbname/_design/titles/_view/by_time?descending=True
And view output would be sorted in reversed way - newest blog articles will go first. Remember that startkey/endkey parameters will limit key range for this reversed order. More about view query parameters you could found in CouchDB wiki
I have three document types MainCategory, Category, SubCategory... each have a parentid which relates to the id of their parent document.
So I want to set up a view so that I can get a list of SubCategories which sit under the MainCategory (preferably just using a map function)... I haven't found a way to arrange the view so this is possible.
I currently have set up a view which gets the following output -
{"total_rows":16,"offset":0,"rows":[
{"id":"11098","key":["22056",0,"11098"],"value":"MainCat...."},
{"id":"11098","key":["22056",1,"11098"],"value":"Cat...."},
{"id":"33610","key":["22056",2,"null"],"value":"SubCat...."},
{"id":"33989","key":["22056",2,"null"],"value":"SubCat...."},
{"id":"11810","key":["22245",0,"11810"],"value":"MainCat...."},
{"id":"11810","key":["22245",1,"11810"],"value":"Cat...."},
{"id":"33106","key":["22245",2,"null"],"value":"SubCat...."},
{"id":"33321","key":["22245",2,"null"],"value":"SubCat...."},
{"id":"11098","key":["22479",0,"11098"],"value":"MainCat...."},
{"id":"11098","key":["22479",1,"11098"],"value":"Cat...."},
{"id":"11810","key":["22945",0,"11810"],"value":"MainCat...."},
{"id":"11810","key":["22945",1,"11810"],"value":"Cat...."},
{"id":"33123","key":["22945",2,"null"],"value":"SubCat...."},
{"id":"33453","key":["22945",2,"null"],"value":"SubCat...."},
{"id":"33667","key":["22945",2,"null"],"value":"SubCat...."},
{"id":"33987","key":["22945",2,"null"],"value":"SubCat...."}
]}
Which QueryString parameters would I use to get say the rows which have a key that starts with ["22945".... When all I have (at query time) is the id "11810" (at query time I don't have knowledge of the id "22945").
If any of that makes sense.
Thanks
The way you store your categories seems to be suboptimal for the query you try to perform on it.
MongoDB.org has a page on various strategies to implement tree-structures (they should apply to Couch and other doc dbs as well) - you should consider Array of Ancestors, where you always store the full path to your node. This makes updating/moving categories more difficult, but querying is easy and fast.