Related
I want to design a question structure with some comments. Which relationship should I use for comments: embed or reference?
A question with some comments, like stackoverflow, would have a structure like this:
Question
title = 'aaa'
content = 'bbb'
comments = ???
At first, I thought of using embedded comments (I think embed is recommended in MongoDB), like this:
Question
title = 'aaa'
content = 'bbb'
comments = [ { content = 'xxx', createdAt = 'yyy'},
{ content = 'xxx', createdAt = 'yyy'},
{ content = 'xxx', createdAt = 'yyy'} ]
It is clear, but I'm worried about this case: If I want to edit a specified comment, how do I get its content and its question? There is no _id to let me find one, nor question_ref to let me find its question. (Is there perhaps a way to do this without _id and question_ref?)
Do I have to use ref rather than embed? Do I then have to create a new collection for comments?
This is more an art than a science. The Mongo Documentation on Schemas is a good reference, but here are some things to consider:
Put as much in as possible
The joy of a Document database is that it eliminates lots of Joins. Your first instinct should be to place as much in a single document as you can. Because MongoDB documents have structure, and because you can efficiently query within that structure (this means that you can take the part of the document that you need, so document size shouldn't worry you much) there is no immediate need to normalize data like you would in SQL. In particular any data that is not useful apart from its parent document should be part of the same document.
Separate data that can be referred to from multiple places into its own collection.
This is not so much a "storage space" issue as it is a "data consistency" issue. If many records will refer to the same data it is more efficient and less error prone to update a single record and keep references to it in other places.
Document size considerations
MongoDB imposes a 4MB (16MB with 1.8) size limit on a single document. In a world of GB of data this sounds small, but it is also 30 thousand tweets or 250 typical Stack Overflow answers or 20 flicker photos. On the other hand, this is far more information than one might want to present at one time on a typical web page. First consider what will make your queries easier. In many cases concern about document sizes will be premature optimization.
Complex data structures:
MongoDB can store arbitrary deep nested data structures, but cannot search them efficiently. If your data forms a tree, forest or graph, you effectively need to store each node and its edges in a separate document. (Note that there are data stores specifically designed for this type of data that one should consider as well)
It has also been pointed out than it is impossible to return a subset of elements in a document. If you need to pick-and-choose a few bits of each document, it will be easier to separate them out.
Data Consistency
MongoDB makes a trade off between efficiency and consistency. The rule is changes to a single document are always atomic, while updates to multiple documents should never be assumed to be atomic. There is also no way to "lock" a record on the server (you can build this into the client's logic using for example a "lock" field). When you design your schema consider how you will keep your data consistent. Generally, the more that you keep in a document the better.
For what you are describing, I would embed the comments, and give each comment an id field with an ObjectID. The ObjectID has a time stamp embedded in it so you can use that instead of created at if you like.
In general, embed is good if you have one-to-one or one-to-many relationships between entities, and reference is good if you have many-to-many relationships.
Well, I'm a bit late but still would like to share my way of schema creation.
I have schemas for everything that can be described by a word, like you would do it in the classical OOP.
E.G.
Comment
Account
User
Blogpost
...
Every schema can be saved as a Document or Subdocument, so I declare this for each schema.
Document:
Can be used as a reference. (E.g. the user made a comment -> comment has a "made by" reference to user)
Is a "Root" in you application. (E.g. the blogpost -> there is a page about the blogpost)
Subdocument:
Can only be used once / is never a reference. (E.g. Comment is saved in the blogpost)
Is never a "Root" in you application. (The comment just shows up in the blogpost page but the page is still about the blogpost)
I came across this small presentation while researching this question on my own. I was surprised at how well it was laid out, both the info and the presentation of it.
http://openmymind.net/Multiple-Collections-Versus-Embedded-Documents
It summarized:
As a general rule, if you have a lot of [child documents] or if they are large, a separate collection might be best.
Smaller and/or fewer documents tend to be a natural fit for embedding.
Actually, I'm quite curious why nobody spoke about the UML specifications. A rule of thumb is that if you have an aggregation, then you should use references. But if it is a composition, then the coupling is stronger, and you should use embedded documents.
And you will quickly understand why it is logical. If an object can exist independently of the parent, then you will want to access it even if the parent doesn't exist. As you just can't embed it in a non-existing parent, you have to make it live in it's own data structure. And if a parent exist, just link them together by adding a ref of the object in the parent.
Don't really know what is the difference between the two relationships ?
Here is a link explaining them:
Aggregation vs Composition in UML
If I want to edit a specified comment, how to get its content and its question?
You can query by sub-document: db.question.find({'comments.content' : 'xxx'}).
This will return the whole Question document. To edit the specified comment, you then have to find the comment on the client, make the edit and save that back to the DB.
In general, if your document contains an array of objects, you'll find that those sub-objects will need to be modified client side.
Yes, we can use the reference in the document. To populate another document just like SQL i joins. In MongoDB, they don't have joins to map one to many relationship documents. Instead that we can use populate to fulfil our scenario.
var mongoose = require('mongoose')
, Schema = mongoose.Schema
var personSchema = Schema({
_id : Number,
name : String,
age : Number,
stories : [{ type: Schema.Types.ObjectId, ref: 'Story' }]
});
var storySchema = Schema({
_creator : { type: Number, ref: 'Person' },
title : String,
fans : [{ type: Number, ref: 'Person' }]
});
The population is the process of automatically replacing the specified paths in the document with the document(s) from other collection(s). We may populate a single document, multiple documents, plain objects, multiple plain objects, or all objects returned from a query. Let's look at some examples.
Better you can get more information please visit: http://mongoosejs.com/docs/populate.html
I know this is quite old but if you are looking for the answer to the OP's question on how to return only specified comment, you can use the $ (query) operator like this:
db.question.update({'comments.content': 'xxx'}, {'comments.$': true})
MongoDB gives freedom to be schema-less and this feature can result in pain in the long term if not thought or planned well,
There are 2 options either Embed or Reference. I will not go through definitions as the above answers have well defined them.
When embedding you should answer one question is your embedded document going to grow, if yes then how much (remember there is a limit of 16 MB per document) So if you have something like a comment on a post, what is the limit of comment count, if that post goes viral and people start adding comments. In such cases, reference could be a better option (but even reference can grow and reach 16 MB limit).
So how to balance it, the answer is a combination of different patterns, check these links, and create your own mix and match based on your use case.
https://www.mongodb.com/blog/post/building-with-patterns-a-summary
https://www.mongodb.com/blog/post/6-rules-of-thumb-for-mongodb-schema-design-part-1
If I want to edit a specified comment, how do I get its content and
its question?
If you had kept track of the number of comments and the index of the comment you wanted to alter, you could use the dot operator (SO example).
You could do f.ex.
db.questions.update(
{
"title": "aaa"
},
{
"comments.0.contents": "new text"
}
)
(as another way to edit the comments inside the question)
I want to design a question structure with some comments. Which relationship should I use for comments: embed or reference?
A question with some comments, like stackoverflow, would have a structure like this:
Question
title = 'aaa'
content = 'bbb'
comments = ???
At first, I thought of using embedded comments (I think embed is recommended in MongoDB), like this:
Question
title = 'aaa'
content = 'bbb'
comments = [ { content = 'xxx', createdAt = 'yyy'},
{ content = 'xxx', createdAt = 'yyy'},
{ content = 'xxx', createdAt = 'yyy'} ]
It is clear, but I'm worried about this case: If I want to edit a specified comment, how do I get its content and its question? There is no _id to let me find one, nor question_ref to let me find its question. (Is there perhaps a way to do this without _id and question_ref?)
Do I have to use ref rather than embed? Do I then have to create a new collection for comments?
This is more an art than a science. The Mongo Documentation on Schemas is a good reference, but here are some things to consider:
Put as much in as possible
The joy of a Document database is that it eliminates lots of Joins. Your first instinct should be to place as much in a single document as you can. Because MongoDB documents have structure, and because you can efficiently query within that structure (this means that you can take the part of the document that you need, so document size shouldn't worry you much) there is no immediate need to normalize data like you would in SQL. In particular any data that is not useful apart from its parent document should be part of the same document.
Separate data that can be referred to from multiple places into its own collection.
This is not so much a "storage space" issue as it is a "data consistency" issue. If many records will refer to the same data it is more efficient and less error prone to update a single record and keep references to it in other places.
Document size considerations
MongoDB imposes a 4MB (16MB with 1.8) size limit on a single document. In a world of GB of data this sounds small, but it is also 30 thousand tweets or 250 typical Stack Overflow answers or 20 flicker photos. On the other hand, this is far more information than one might want to present at one time on a typical web page. First consider what will make your queries easier. In many cases concern about document sizes will be premature optimization.
Complex data structures:
MongoDB can store arbitrary deep nested data structures, but cannot search them efficiently. If your data forms a tree, forest or graph, you effectively need to store each node and its edges in a separate document. (Note that there are data stores specifically designed for this type of data that one should consider as well)
It has also been pointed out than it is impossible to return a subset of elements in a document. If you need to pick-and-choose a few bits of each document, it will be easier to separate them out.
Data Consistency
MongoDB makes a trade off between efficiency and consistency. The rule is changes to a single document are always atomic, while updates to multiple documents should never be assumed to be atomic. There is also no way to "lock" a record on the server (you can build this into the client's logic using for example a "lock" field). When you design your schema consider how you will keep your data consistent. Generally, the more that you keep in a document the better.
For what you are describing, I would embed the comments, and give each comment an id field with an ObjectID. The ObjectID has a time stamp embedded in it so you can use that instead of created at if you like.
In general, embed is good if you have one-to-one or one-to-many relationships between entities, and reference is good if you have many-to-many relationships.
Well, I'm a bit late but still would like to share my way of schema creation.
I have schemas for everything that can be described by a word, like you would do it in the classical OOP.
E.G.
Comment
Account
User
Blogpost
...
Every schema can be saved as a Document or Subdocument, so I declare this for each schema.
Document:
Can be used as a reference. (E.g. the user made a comment -> comment has a "made by" reference to user)
Is a "Root" in you application. (E.g. the blogpost -> there is a page about the blogpost)
Subdocument:
Can only be used once / is never a reference. (E.g. Comment is saved in the blogpost)
Is never a "Root" in you application. (The comment just shows up in the blogpost page but the page is still about the blogpost)
I came across this small presentation while researching this question on my own. I was surprised at how well it was laid out, both the info and the presentation of it.
http://openmymind.net/Multiple-Collections-Versus-Embedded-Documents
It summarized:
As a general rule, if you have a lot of [child documents] or if they are large, a separate collection might be best.
Smaller and/or fewer documents tend to be a natural fit for embedding.
Actually, I'm quite curious why nobody spoke about the UML specifications. A rule of thumb is that if you have an aggregation, then you should use references. But if it is a composition, then the coupling is stronger, and you should use embedded documents.
And you will quickly understand why it is logical. If an object can exist independently of the parent, then you will want to access it even if the parent doesn't exist. As you just can't embed it in a non-existing parent, you have to make it live in it's own data structure. And if a parent exist, just link them together by adding a ref of the object in the parent.
Don't really know what is the difference between the two relationships ?
Here is a link explaining them:
Aggregation vs Composition in UML
If I want to edit a specified comment, how to get its content and its question?
You can query by sub-document: db.question.find({'comments.content' : 'xxx'}).
This will return the whole Question document. To edit the specified comment, you then have to find the comment on the client, make the edit and save that back to the DB.
In general, if your document contains an array of objects, you'll find that those sub-objects will need to be modified client side.
Yes, we can use the reference in the document. To populate another document just like SQL i joins. In MongoDB, they don't have joins to map one to many relationship documents. Instead that we can use populate to fulfil our scenario.
var mongoose = require('mongoose')
, Schema = mongoose.Schema
var personSchema = Schema({
_id : Number,
name : String,
age : Number,
stories : [{ type: Schema.Types.ObjectId, ref: 'Story' }]
});
var storySchema = Schema({
_creator : { type: Number, ref: 'Person' },
title : String,
fans : [{ type: Number, ref: 'Person' }]
});
The population is the process of automatically replacing the specified paths in the document with the document(s) from other collection(s). We may populate a single document, multiple documents, plain objects, multiple plain objects, or all objects returned from a query. Let's look at some examples.
Better you can get more information please visit: http://mongoosejs.com/docs/populate.html
I know this is quite old but if you are looking for the answer to the OP's question on how to return only specified comment, you can use the $ (query) operator like this:
db.question.update({'comments.content': 'xxx'}, {'comments.$': true})
MongoDB gives freedom to be schema-less and this feature can result in pain in the long term if not thought or planned well,
There are 2 options either Embed or Reference. I will not go through definitions as the above answers have well defined them.
When embedding you should answer one question is your embedded document going to grow, if yes then how much (remember there is a limit of 16 MB per document) So if you have something like a comment on a post, what is the limit of comment count, if that post goes viral and people start adding comments. In such cases, reference could be a better option (but even reference can grow and reach 16 MB limit).
So how to balance it, the answer is a combination of different patterns, check these links, and create your own mix and match based on your use case.
https://www.mongodb.com/blog/post/building-with-patterns-a-summary
https://www.mongodb.com/blog/post/6-rules-of-thumb-for-mongodb-schema-design-part-1
If I want to edit a specified comment, how do I get its content and
its question?
If you had kept track of the number of comments and the index of the comment you wanted to alter, you could use the dot operator (SO example).
You could do f.ex.
db.questions.update(
{
"title": "aaa"
},
{
"comments.0.contents": "new text"
}
)
(as another way to edit the comments inside the question)
Going off an example in the accepted answer here:
Mongo DB relations between objects
For a blogging system, "Posts should be a collection. post author might be a separate collection, or simply a field within posts if only an email address. comments should be embedded objects within a post for performance."
If this is the case, does that mean that every time my app displays a blog post, I'm loading every single comment that was ever made on that post? What if there are 3,729 comments? Wouldn't this brutalize the database connection, SQL or NoSQL? Also there's the obvious scenario in which when I load a blog post, I want to show only the first 10 comments initially.
Document databases are not relational databases. You CANNOT first build the database model and then later on decide on various interesting ways of querying it. Instead, you should first determine what access patterns you want to support, and then design the document schemas accordingly.
So in order to answer your question, what we really need to know is how you intend to use the data. Displaying comments associated with a post is a distinctly different scenario than displaying all comments from a particular author. Each one of those requirements will dictate a different design, as will supporting them both.
This in itself may be useful information to you (?), but I suspect you want more concrete answers :) So please add some additional details on your intended usage.
Adding more info:
There are a few "do" and "don'ts" when deciding on a strategy:
DO: Optimize for the common use-cases. There is often a 20/80 breakdown where 20% of the UX drives 80% of the load - the homepage/landing page is a classic example. First priority is to make sure that these are as efficient as possible. Make sure that your data model allows either A) loading those in either a single IO request or B) is cache-friendly
DONT: don't fall into the dreaded "N+1" trap. This pattern occurs when you data model forces you to make N calls in order to load N entities, often preceded by an additional call to get the list of the N IDs. This is a killer, especially together with #3...
DO: Always cap (via the UX) the amount of data which you are willing to fetch. If the user has 3729 comments you obviously aren't going to fetch them all at once. Even it it was feasible from a database perspective, the user experience would be horrible. Thats why search engines use the "next 20 results" paradigm. So you can (for example) align the database structure to the UX and save the comments in blocks of 20. Then each page refresh involves a single DB get.
DO: Balance the Read and Write requirements. Some types of systems are read-heavy and you can assume that for each write there will be many reads (StackOverflow is a good example). So there it makes sense to make writes more expensive in order to gain benefits in read performance. For example, data denormalization and duplication. Other systems are evenly balanced or even write heavy and require other approaches
DO: Use the dimension of TIME to your advantage. Twitter is a classic example: 99.99% of tweets will never be accessed after the first hour/day/week/whatever. That opens all kinds of interesting optimization possibilities in the your data schema.
This is just the tip of the iceberg. I suggest reading up a little on column-based NoSQL systems (such as Cassandra)
Not sure if this answers you question, but anyhow you can throttle the amount of blog comments in two ways:
Load only the last 10 , or range of blog comments using $slice operator
db.blogs.find( {_id : someValue}, { comments: { $slice: -10 } } )
will return last 10 comments
db.blogs.find( {_id : someValue}, { comments: { $slice: [-10, 10] } } )
will return next 10 comments
Use capped array to save only the last n blog posts using capped arrays
Hi I'm new to mongoDB and Mongoid and am little bit confused on when to use embedded documents and how deep to embedd.
So a fictional example:
Library collection has_many :books, which embeds_many :pages, which embeds_many :sections
Since I cannot work with say a Section directly I have to go trough books.pages.sections, right?
This would result in this route, libraries/:id/books/:id/pages/:id/sections/:id
Which seems a little bit crazy, best practice would be to only nest one level deep, right?
One way would be to have the route pages/:id/sections/:id and then stick the bookid in the request?
Would it be harder to query on say, sections? For example if I need to find all the books where sections has tag x?
However if I don't embedd all the way I would have an extra query?
Can someone shed some light? Thanks.
First of I believe that no one can give you the right answer about how deeply you should embed documents. It is highly dependent on your concrete project requirements. In general you should answer some questions to choose appropriate schema design:
Will users concurrently update same object in collection? (or what would my boss say if clients lost their updates)
Do I need support atomic operations?
Do I need to independently show nested collections or are they queried with the parent?
Do I need to sort embedded objects?
Do I need to query on embedded objects?
If you will answer 1,2-true; 3 I need show them independently (different page); 4,5 - true then i am sure that embedding will be some kind of pain in your ass.
Extra queries should not be a problem I guess.
I want to design a question structure with some comments. Which relationship should I use for comments: embed or reference?
A question with some comments, like stackoverflow, would have a structure like this:
Question
title = 'aaa'
content = 'bbb'
comments = ???
At first, I thought of using embedded comments (I think embed is recommended in MongoDB), like this:
Question
title = 'aaa'
content = 'bbb'
comments = [ { content = 'xxx', createdAt = 'yyy'},
{ content = 'xxx', createdAt = 'yyy'},
{ content = 'xxx', createdAt = 'yyy'} ]
It is clear, but I'm worried about this case: If I want to edit a specified comment, how do I get its content and its question? There is no _id to let me find one, nor question_ref to let me find its question. (Is there perhaps a way to do this without _id and question_ref?)
Do I have to use ref rather than embed? Do I then have to create a new collection for comments?
This is more an art than a science. The Mongo Documentation on Schemas is a good reference, but here are some things to consider:
Put as much in as possible
The joy of a Document database is that it eliminates lots of Joins. Your first instinct should be to place as much in a single document as you can. Because MongoDB documents have structure, and because you can efficiently query within that structure (this means that you can take the part of the document that you need, so document size shouldn't worry you much) there is no immediate need to normalize data like you would in SQL. In particular any data that is not useful apart from its parent document should be part of the same document.
Separate data that can be referred to from multiple places into its own collection.
This is not so much a "storage space" issue as it is a "data consistency" issue. If many records will refer to the same data it is more efficient and less error prone to update a single record and keep references to it in other places.
Document size considerations
MongoDB imposes a 4MB (16MB with 1.8) size limit on a single document. In a world of GB of data this sounds small, but it is also 30 thousand tweets or 250 typical Stack Overflow answers or 20 flicker photos. On the other hand, this is far more information than one might want to present at one time on a typical web page. First consider what will make your queries easier. In many cases concern about document sizes will be premature optimization.
Complex data structures:
MongoDB can store arbitrary deep nested data structures, but cannot search them efficiently. If your data forms a tree, forest or graph, you effectively need to store each node and its edges in a separate document. (Note that there are data stores specifically designed for this type of data that one should consider as well)
It has also been pointed out than it is impossible to return a subset of elements in a document. If you need to pick-and-choose a few bits of each document, it will be easier to separate them out.
Data Consistency
MongoDB makes a trade off between efficiency and consistency. The rule is changes to a single document are always atomic, while updates to multiple documents should never be assumed to be atomic. There is also no way to "lock" a record on the server (you can build this into the client's logic using for example a "lock" field). When you design your schema consider how you will keep your data consistent. Generally, the more that you keep in a document the better.
For what you are describing, I would embed the comments, and give each comment an id field with an ObjectID. The ObjectID has a time stamp embedded in it so you can use that instead of created at if you like.
In general, embed is good if you have one-to-one or one-to-many relationships between entities, and reference is good if you have many-to-many relationships.
Well, I'm a bit late but still would like to share my way of schema creation.
I have schemas for everything that can be described by a word, like you would do it in the classical OOP.
E.G.
Comment
Account
User
Blogpost
...
Every schema can be saved as a Document or Subdocument, so I declare this for each schema.
Document:
Can be used as a reference. (E.g. the user made a comment -> comment has a "made by" reference to user)
Is a "Root" in you application. (E.g. the blogpost -> there is a page about the blogpost)
Subdocument:
Can only be used once / is never a reference. (E.g. Comment is saved in the blogpost)
Is never a "Root" in you application. (The comment just shows up in the blogpost page but the page is still about the blogpost)
I came across this small presentation while researching this question on my own. I was surprised at how well it was laid out, both the info and the presentation of it.
http://openmymind.net/Multiple-Collections-Versus-Embedded-Documents
It summarized:
As a general rule, if you have a lot of [child documents] or if they are large, a separate collection might be best.
Smaller and/or fewer documents tend to be a natural fit for embedding.
Actually, I'm quite curious why nobody spoke about the UML specifications. A rule of thumb is that if you have an aggregation, then you should use references. But if it is a composition, then the coupling is stronger, and you should use embedded documents.
And you will quickly understand why it is logical. If an object can exist independently of the parent, then you will want to access it even if the parent doesn't exist. As you just can't embed it in a non-existing parent, you have to make it live in it's own data structure. And if a parent exist, just link them together by adding a ref of the object in the parent.
Don't really know what is the difference between the two relationships ?
Here is a link explaining them:
Aggregation vs Composition in UML
If I want to edit a specified comment, how to get its content and its question?
You can query by sub-document: db.question.find({'comments.content' : 'xxx'}).
This will return the whole Question document. To edit the specified comment, you then have to find the comment on the client, make the edit and save that back to the DB.
In general, if your document contains an array of objects, you'll find that those sub-objects will need to be modified client side.
Yes, we can use the reference in the document. To populate another document just like SQL i joins. In MongoDB, they don't have joins to map one to many relationship documents. Instead that we can use populate to fulfil our scenario.
var mongoose = require('mongoose')
, Schema = mongoose.Schema
var personSchema = Schema({
_id : Number,
name : String,
age : Number,
stories : [{ type: Schema.Types.ObjectId, ref: 'Story' }]
});
var storySchema = Schema({
_creator : { type: Number, ref: 'Person' },
title : String,
fans : [{ type: Number, ref: 'Person' }]
});
The population is the process of automatically replacing the specified paths in the document with the document(s) from other collection(s). We may populate a single document, multiple documents, plain objects, multiple plain objects, or all objects returned from a query. Let's look at some examples.
Better you can get more information please visit: http://mongoosejs.com/docs/populate.html
I know this is quite old but if you are looking for the answer to the OP's question on how to return only specified comment, you can use the $ (query) operator like this:
db.question.update({'comments.content': 'xxx'}, {'comments.$': true})
MongoDB gives freedom to be schema-less and this feature can result in pain in the long term if not thought or planned well,
There are 2 options either Embed or Reference. I will not go through definitions as the above answers have well defined them.
When embedding you should answer one question is your embedded document going to grow, if yes then how much (remember there is a limit of 16 MB per document) So if you have something like a comment on a post, what is the limit of comment count, if that post goes viral and people start adding comments. In such cases, reference could be a better option (but even reference can grow and reach 16 MB limit).
So how to balance it, the answer is a combination of different patterns, check these links, and create your own mix and match based on your use case.
https://www.mongodb.com/blog/post/building-with-patterns-a-summary
https://www.mongodb.com/blog/post/6-rules-of-thumb-for-mongodb-schema-design-part-1
If I want to edit a specified comment, how do I get its content and
its question?
If you had kept track of the number of comments and the index of the comment you wanted to alter, you could use the dot operator (SO example).
You could do f.ex.
db.questions.update(
{
"title": "aaa"
},
{
"comments.0.contents": "new text"
}
)
(as another way to edit the comments inside the question)