Foreign keys in mongo

Question

How do I design a scheme such this in MongoDB  I think there are no foreign keys

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Short answer  You should to use  quot weak references quot  between collections  using ObjectId properties   References store the relationships between data by including links or references from one document to another  Applications can resolve these references to access the related data  Broadly  these are normalized data models   https   docs mongodb com manual core data-modeling-introduction  references This will of course not check any referential integrity  You need to handle  quot dead links quot  on your side  application level

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The purpose of ForeignKey is to prevent the creation of data if the field value does not match its ForeignKey  To accomplish this in MongoDB  we use Schema middlewares that ensure the data consistency  Please have a look at the documentation  https   mongoosejs com docs middleware html pre

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How to design table like this in mongodb    First  to clarify some naming conventions  MongoDB uses collections instead of tables      I think there are no foreign keys    Take the following model   student       id  ObjectId         name   Jane     courses          course   bio101   mark  85          course   chem101   mark  89          course      id   bio101     name   Biology 101     description   Introduction to biology      Clearly Jane s course list points to some specific courses  The database does not apply any constraints to the system  i e   foreign key constraints   so there are no  cascading deletes  or  cascading updates   However  the database does contain the correct information   In addition  MongoDB has a DBRef standard that helps standardize the creation of these references  In fact  if you take a look at that link  it has a similar example      How can I solve this task    To be clear  MongoDB is not relational  There is no standard  normal form   You should model your database appropriate to the data you store and the queries you intend to run

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We can define the so-called foreign key in MongoDB  However  we need to maintain the data integrity BY OURSELVES  For example   student       id  ObjectId         name   Jane     courses    bio101    bio102         lt   ids of the courses    course      id   bio101     name   Biology 101     description   Introduction to biology      The courses field contains  ids of courses  It is easy to define a one-to-many relationship  However  if we want to retrieve the course names of student Jane  we need to perform another operation to retrieve the course document via  id    If the course bio101 is removed  we need to perform another operation to update the courses field in the student document   More  MongoDB Schema Design  The document-typed nature of MongoDB supports flexible ways to define relationships  To define a one-to-many relationship   Embedded document   Suitable for one-to-few   Advantage  no need to perform additional queries to another document  Disadvantage  cannot manage the entity of embedded documents individually    Example   student     name   Kate Monster     addresses            street   123 Sesame St   city   Anytown   cc   USA            street   123 Avenue Q   city   New York   cc   USA            Child referencing  Like the student course example above   Parent referencing  Suitable for one-to-squillions  such as log messages   host        id   ObjectID  AAAB        name    goofy example com       ipaddr    127 66 66 66     logmsg       time   ISODate  2014-03-28T09 42 41 382Z        message    cpu is on fire        host  ObjectID  AAAB            Reference to the Host document     Virtually  a host is the parent of a logmsg  Referencing to the host id saves much space given that the log messages are squillions   References    6 Rules of Thumb for MongoDB Schema Design  Part 1 6 Rules of Thumb for MongoDB Schema Design  Part 2 6 Rules of Thumb for MongoDB Schema Design  Part 3 Model One-to-Many Relationships with Document References

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From The Little MongoDB Book     Yet another alternative to using joins is to denormalize your data  Historically  denormalization was reserved for   performance-sensitive code  or when data should be snapshotted  like in an audit log   However  with the ever-   growing popularity of NoSQL  many of which don   t have joins  denormalization as part of normal modeling is becoming   increasingly common  This doesn   t mean you should duplicate every piece of information in every document  However    rather than letting fear of duplicate data drive your design decisions  consider modeling your data based on what   information belongs to what document    So   student         id  ObjectId           name   Jane       courses                   name   Biology 101            mark  85           id bio101                 If its a RESTful API data  replace the course id with a GET link to the course resource

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You may be interested in using a ORM like Mongoid or MongoMapper   http   mongoid org docs relations referenced 1-n html  In a NoSQL database like MongoDB there are not  tables  but collections  Documents are grouped inside Collections  You can have any kind of document     with any kind of data     in a single collection  Basically  in a NoSQL database it is up to you to decide how to organise the data and its relations  if there are any   What Mongoid and MongoMapper do is to provide you with convenient methods to set up relations quite easily  Check out the link I gave you and ask any thing   Edit   In mongoid you will write your scheme like this   class Student   include Mongoid  Document      field  name     embeds many  addresses     embeds many  scores     end  class Address   include Mongoid  Document      field  address     field  city     field  state     field  postalCode     embedded in  student end  class Score   include Mongoid  Document      belongs to  course     field  grade  type  Float     embedded in  student end   class Course   include Mongoid  Document    field  name   has many  scores   end   Edit    gt  db foo insert  group  phones     gt  db foo find                         id    ObjectId  4df6539ae90592692ccc9940     group     phones        id    ObjectId  4df6540fe90592692ccc9941     group     phones     gt db foo find    id  ObjectId  4df6539ae90592692ccc9940          id    ObjectId  4df6539ae90592692ccc9940     group     phones      You can use that ObjectId in order to do relations between documents

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