I need to find all documents in a MongoDB database that have a property containing a string that is similar to the search term but allows for a certain % in divergence.
In plain javascript I could for example use https://www.npmjs.com/package/string-similarity and then basically match all documents that have > 90% similarity score.
I'd like do to this as MongoDB query and be as performant as possible as the database contains millions of documents.
What possible options do I have in this situation?
I found something about $text search, but it doesn't seem it helps a lot
I was thinking about creating some sort of signature for each document, like some sort of hash that allows for some sort of divergence.
I am really happy for every idea to get this solved in the best possible way.
The common solution to this problem is to use a search engine database, like Elasticsearch or Atlas search (by Mongodb team). I will not go into too much detail on how these databases work but generally speaking they are an inverse index database, this means you tokenize your data on insert and then your queries run on the tokenized data and not on the raw data set.
This approach is very powerful and can help with many "search engine" problems like autocomplete or in your case what is called a "fuzzy" search.
Let's see how elasticsearch deals with this by reading about their fuzzy feature:
To find similar terms, the fuzzy query creates a set of all possible variations, or expansions, of the search term within a specified edit distance. The query then returns exact matches for each expansion.
Basically what they do is create all "possible" permutations of the query within the given parameters. I would personally recommend you just use one of these databases that give this ability OOTB, however if you want to do a "pseudo" search engine in Mongo you can just use this approach ( with the downside of Mongo's indexes being a tree so you force a tree scan for these queries instead of a db designed for this )
I wonder if there's a way to limit the number of elements in a (sub)document. In my app, elements can be added to the subdocument with ajax, so its important that I prevent adding a ridiculous number of entries by a malicious user.
I can think of one way, that is to query the parent document, check number of elements in the subdocument and if its less than X, save it. That however requires to query the whole document, just to update a little thing. I would rather much prefer to do everything with update(), instead of findOne() and save(). Is there a way?
Edit: It can be done without db lookup prior to updating. I don't know if it's possible to do with a subdoc, I did it with a plain object. Simply store your data in an array or better a classical object. Then before updating, validate key numbers, if key number is greater than the allowed - ignore it. Ofc if dealing with an object, check if the key is a number first.
You can make an array limit in Mongoose, using a custom validate function, with something like:
validate: [v => v.length <= MAX, 'message']
where MAX is some maximal number of elements, and 'message' is a message to show on validation failure. See:
http://mongoosejs.com/docs/api.html#schematype_SchemaType-validate
But you cannot make a one request for MongoDB that would fail if the array is too long because MongoDB has no concept of schemas and that would be needed to do what you want. Schemas are only present in Mongoose.
So if you want to use MongoDB directly then not only you will not be able to do it in one request, it's actually much worse than that - you also cannot eliminate a race condition because MongoDB doesn't support transactions, so you cannot really guarantee that your limit will never be exceeded.
So I'm using mongodb and I'm unsure if I've got the correct / best database collection design for what I'm trying to do.
There can be many items, and a user can create new groups with these items in. Any user may follow any group!
I have not just added the followers and items into the group collection because there could be 5 items in the group, or there could be 10000 (and the same for followers) and from research I believe that you should not use unbound arrays (where the limit is unknown) due to performance issues when the document has to be moved because of its expanding size. (Is there a recommended maximum for array lengths before hitting performance issues anyway?)
I think with the following design a real performance issue could be when I want to get all of the groups that a user is following for a specific item (based off of the user_id and item_id), because then I have to find all of the groups the user is following, and from that find all of the item_groups with the group_id $in and the item id. (but I can't actually see any other way of doing this)
Follower
.find({ user_id: "54c93d61596b62c316134d2e" })
.exec(function (err, following) {
if (err) {throw err;};
var groups = [];
for(var i = 0; i<following.length; i++) {
groups.push(following[i].group_id)
}
item_groups.find({
'group_id': { $in: groups },
'item_id': '54ca9a2a6508ff7c9ecd7810'
})
.exec(function (err, groups) {
if (err) {throw err;};
res.json(groups);
});
})
Are there any better DB patterns for dealing with this type of setup?
UPDATE: Example use case added in comment below.
Any help / advice will be really appreciated.
Many Thanks,
Mac
I agree with the general notion of other answers that this is a borderline relational problem.
The key to MongoDB data models is write-heaviness, but that can be tricky for this use case, mostly because of the bookkeeping that would be required if you wanted to link users to items directly (a change to a group that is followed by lots of users would incur a huge number of writes, and you need some worker to do this).
Let's investigate whether the read-heavy model is inapplicable here, or whether we're doing premature optimization.
The Read Heavy Approach
Your key concern is the following use case:
a real performance issue could be when I want to get all of the groups that a user is following for a specific item [...] because then I have to find all of the groups the user is following, and from that find all of the item_groups with the group_id $in and the item id.
Let's dissect this:
Get all groups that the user is following
That's a simple query: db.followers.find({userId : userId}). We're going to need an index on userId which will make the runtime of this operation O(log n), or blazing fast even for large n.
from that find all of the item_groups with the group_id $in and the item id
Now this the trickier part. Let's assume for a moment that it's unlikely for items to be part of a large number of groups. Then a compound index { itemId, groupId } would work best, because we can reduce the candidate set dramatically through the first criterion - if an item is shared in only 800 groups and the user is following 220 groups, mongodb only needs to find the intersection of these, which is comparatively easy because both sets are small.
We'll need to go deeper than this, though:
The structure of your data is probably that of a complex network. Complex networks come in many flavors, but it makes sense to assume your follower graph is nearly scale-free, which is also pretty much the worst case. In a scale free network, a very small number of nodes (celebrities, super bowl, Wikipedia) attract a whole lot of 'attention' (i.e. have many connections), while a much larger number of nodes have trouble getting the same amount of attention combined.
The small nodes are no reason for concern, the queries above, including round-trips to the database are in the 2ms range on my development machine on a dataset with tens of millions of connections and > 5GB of data. Now that data set isn't huge, but no matter what technology you choose you, will be RAM bound because the indices must be in RAM in any case (data locality and separability in networks is generally poor), and the set intersection size is small by definition. In other words: this regime is dominated by hardware bottlenecks.
What about the supernodes though?
Since that would be guesswork and I'm interested in network models a lot, I took the liberty of implementing a dramatically simplified network tool based on your data model to make some measurements. (Sorry it's in C#, but generating well-structured networks is hard enough in the language I'm most fluent in...).
When querying the supernodes, I get results in the range of 7ms tops (that's on 12M entries in a 1.3GB db, with the largest group having 133,000 items in it and a user that follows 143 groups.)
The assumption in this code is that the number of groups followed by a user isn't huge, but that seems reasonable here. If it's not, I'd go for the write-heavy approach.
Feel free to play with the code. Unfortunately, it will need a bit of optimization if you want to try this with more than a couple of GB of data, because it's simply not optimized and does some very inefficient calculations here and there (especially the beta-weighted random shuffle could be improved).
In other words: I wouldn't worry about the performance of the read-heavy approach yet. The problem is often not so much that the number of users grows, but that users use the system in unexpected ways.
The Write Heavy Approach
The alternative approach is probably to reverse the order of linking:
UserItemLinker
{
userId,
itemId,
groupIds[] // for faster retrieval of the linker. It's unlikely that this grows large
}
This is probably the most scalable data model, but I wouldn't go for it unless we're talking about HUGE amounts of data where sharding is a key requirement. The key difference here is that we can now efficiently compartmentalize the data by using the userId as part of the shard key. That helps to parallelize queries, shard efficiently and improve data locality in multi-datacenter-scenarios.
This could be tested with a more elaborate version of the testbed, but I didn't find the time yet, and frankly, I think it's overkill for most applications.
I read your comment/use-case. So I update my answer.
I suggest to change the design as per this article: MongoDB Many-To-Many
The design approach is different and you might want to remodel your approach to this. I'll try to give you an idea to start with.
I make the assumption that a User and a Follower are basically the same entities here.
I think the point you might find interesting is that in MongoDB you can store array fields and this is what I will use to simplify/correct your design for MongoDB.
The two entities I would omit are: Followers and ItemGroups
Followers: It is simply a User who can follow Groups. I would add an
array of group ids to have a list of Groups that the User follows. So instead of having an entity Follower, I would only have User with an array field that has a list of Group Ids.
ItemGroups: I would remove this entity too. Instead I would use an array of Item Ids in the Group entity and an array of Group Ids in the Item entity.
This is basically it. You will be able to do what you described in your use case. The design is simpler and more accurate in the sense that it reflects the design decisions of a document based database.
Notes:
You can define indexes on array fields in MongoDB. See Multikey Indexes for example.
Be wary about using indexes on array fields though. You need to understand your use case in order to decide whether it is reasonable or not. See this article. Since you only reference ObjectIds I thought you could try it, but there might be other cases where it is better to change the design.
Also note that the ID field _id is a MongoDB
specific field type of ObjectID used as primary key. To access the ids you can refer to it e.g. as user.id, group.id, etc. You can use an index to ensure uniqueness as per this question.
Your schema design could look like this:
As to your other question/concerns
Is there a recommended maximum for array lengths before hitting performance issues anyway?
the answer is in MongoDB the document size is limited to 16 MB and there is now way you can work around that. However 16 MB is considered to be sufficient; if you hit the 16 MB then your design has to be improved. See here for info, section Document Size Limit.
I think with the following design a real performance issue could be when I want to get all of the groups that a user is following for a specific item (based off of the user_id and item_id)...
I would do this way. Note how "easier" it sounds when using MongoDB.
get the item of the user
get groups that reference that item
I would be rather concerned if the arrays get very large and you are using indexes on them. This could overall slow down write operations on the respective document(s). Maybe not so much in your case, but not entirely sure.
You're on the right track to creating a performant NoSQL schema design, and I think you're asking the right questions as to how to properly lay things out.
Here's my understanding of your application:
It looks like Groups can both have many Followers (mapping users to groups) and many Items, but Items may not necessarily be in many Groups (although it is possible). And from your given use-case example, it sounds like retrieving all the Groups an Item is in and all the Items in a Group will be some common read operations.
In your current schema design, you've implemented a model between mapping users to groups as followers and items to groups as item_groups. This works alright until you mention the problem with more complex queries:
I think with the following design a real performance issue could be when I want to get all of the groups that a user is following for a specific item (based off of the user_id and item_id)
I think a few things could help you out in this situation:
Take advantage of MongoDB's powerful indexing capabilities. In particular, I think you should consider creating compound indexes on your Follower objects covering your Group and User, and your Item_Groups on Item and Group, respectively. You'll also want to make sure this kind of relationship is unique, in that a user can only follow a group once and an item can only be added to a group once. This would best be achieved in some pre-save hooks defined in your schema, or using a plugin to check for validity.
FollowerSchema.index({ group: 1, user: 1 }, { unique: true });
Item_GroupsSchema.index({ group: 1, item: 1 }, { unique: true });
Using an index on these fields will create some overhead when writing to the collection, but it sounds like reading from the collection will be a more common interaction so it'll be worth it (I'd suggest reading more up on index performance).
Since a User probably won't be following thousands of groups, I think it'd be worthwhile to include in the user model an array of groups the user is following. This will help you out with that complex query when you want to find all instances of an item in groups that a user is currently following, since you'll have the list of groups right there. You'll still have the implementation where your using $in: groups, but it'll be with one less query to the collection.
As I mentioned before, it seems like items may not necessarily be in that many groups (just like users won't necessarily be following thousands of groups). If the case may commonly be that an item is in maybe a couple hundred groups, I'd consider just adding an array to the item model for each group that it gets added to. This would increase your performance when reading all the groups an item is in, a query you mentioned would be a common one. Note: You'd still use the Item_Groups model to retrieve all the items in a group by querying on the (now indexed) group_id.
Unfortunately NoSQL databases aren't eligible in this case. Your data model seems exact relational. According to MongoDB documentation we can do only these and can perform only these.
There are some practices. MongoDB advises to us using Followers collection to get which user follows which group and vice versa with good performance. You may find the closest case to your situation on this page on slide 14th. But I think the slides can eligible if you want to get each result on different page. For instance; You are a twitter user and when you click the followers button you'll see the all your followers. And then you click on a follower name you'll see the follower's messages and whatever you can see. As we can see all of those work step-by-step. No needed a relational query.
I believe that you should not use unbound arrays (where the limit is unknown) due to performance issues when the document has to be moved because of its expanding size. (Is there a recommended maximum for array lengths before hitting performance issues anyway?)
Yes, you're right. http://askasya.com/post/largeembeddedarrays .
But if you have about a hundred items in your array there is no problem.
If you have fixed size some data thousands you may embed them into your relational collections as array. And you can query your indexed embedded document fields rapidly.
In my humble opinion, you should create hundreds of thousands test data and check performances of using embedded documents and arrays eligible to your case. Don't forget creating indexes appropriate your queries. You may try to using document references on your tests. After tests if you like performance of results go ahead..
You had tried to find group_id records that are followed by a specific user and then you've tried to find a specific item with those group_id. Would it be possible Item_Groups and Followers collections have a many-to-many relation?
If so, many-to-many relation isn't supported by NoSQL databases.
Is there any chance you can change your database to MySQL?
If so you should check this out.
briefly MongoDB pros against to MySQL;
- Better writing performance
briefly MongoDB cons against to MySQL;
- Worse reading performance
If you work on Node.js you may check https://www.npmjs.com/package/mysql and https://github.com/felixge/node-mysql/
Good luck...
I am trying to fetch documents from couchdb based on certain specific filters through javascript. For example i need to get the list of employees from a db where the key can be either city, age, state, gross income, gender or a combination of two or more such keys.
The problem i am facing is as the number of possible keys increase the number of views i need to write also increases drastically. I want to avoid writing so many views. So is it possible to do so ??
In addition to checking out Matt's suggestion about couchdb-lucene, you might also look into list functions: they're quite useful when you have a small set of basic view queries that will reduce the number of records fetched to a manageable level and you want to do a bunch of ad-hoc queries that further filter those records.
Long story short : I'd like to treat several javascript associative arrays as a database (where the arrays are tables). The relations could be represented by special fields inside the arrays. I'm not interested in the persistence aspect of a database, I only want to be able to query the arrays with a SQL-like language and retrieve sets of data in the form of associative arrays.
My question : Is there any javascript library that has such features ? Otherwise, is there any library that can at least take care of the SQL-like language part ?
Thanks
I believe the closest thing to what you need is the jLinq library. It can operate with js objects and arrays much in the same way you would do with a database, but in a slightly different way. You don't really write queries, but use methods to construct them. Overall it's way better I think.
Some googling found this: http://ajaxian.com/archives/two-js-solutions-to-run-sql-like-statements-on-arrays-and-objects which seemed interesting.
Can I ask why you want to do this?
I came across this question while searching something sort of related. Wanted to share with you (9 years later, I do realize) that I have the same want/need, often, where the script I'm working on has a lot of cross-referencing to do between various sources of information. I use PowerShell. Enumerating arrays of objects, from within a loop, which is enumerating other objects, is just bad/slow/horrible.
To date, my solution has been to take all my arrays and then make hashtables from them, where the key/name is a property value that is common across all the arrays (e.g. ObjectId (GUID)), and the value is the entire object from the array. With this, while in my loop which is enumerating Array#1, I can check for the presence of this current item in any of the other arrays simply by checking the existence of the key in the corresponding hashtables, and that way there's no enumerating the other array, there's just direct , equal effort access to the correct item in the array (but really coming from the newly built hashtable).
So my arrays are just temporary collection buckets, then everything I do from there uses the hashtables, which are just index/lookup tables.
What I was searching for when I stumbled here, was for solutions to keep track of all the different hashtables in the building/planning phase of my scripts.