MongoDB relationships embed or reference

Question

I m new to MongoDB--coming from a relational database background  I want to design a question structure with some comments  but I don t know which relationship to 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 want to use embeded 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 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   I m so newbie  that I don t know if there s any way to do this without  id and question ref    Do I have to use ref not embed  Then I have to create a new collection for comments

User · Answer

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

User · Answer

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

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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

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Yes  we can use the reference in the document To populate the another document just like sql i joins In mongo db they dont have joins to mapping one to many relationship document Instead that we can use populate to fulfill 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           Population is the process of automatically replacing the specified paths in the document with document s  from other collection s   We may populate a single document  multiple documents  plain object  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

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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

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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

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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  quot storage space quot  issue as it is a  quot data consistency quot  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  quot lock quot  a record on the server  you can build this into the client s logic using for example a  quot lock quot  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

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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

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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

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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

[mongodb] MongoDB relationships: embed or reference?

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