Basic article on relationships in MongoDB, approaches, types of relationships, examples and features.

Recommendations

Before you start studying relationships in MongoDB, it is recommended to have some prior knowledge before reading this article, so I invite you to review the following optional support links:

Agregaciones y tuberías en Mongo DB (Aggregation Operations)

Relationships in MongoDB

One of the criteria that tends to be confused is the term NoSQL databases with the absence of relationships, but the truth is that many of the NoSQL databases already offer us functionalities that allow us to have a certain degree of relationship between the data.

For a couple of years now, MongoDB has allowed the implementation of relationships through a concept called “ references ”. It is worth noting that even the official MongoDB documentation talks about cardinalities and other characteristics that are known in a relational database.

The biggest distinction is that MongoDB does not faithfully adhere to the ER (Entity Relationship) model and other qualities of relational databases.

Relationship Approaches in MongoDB

Relationships in MongoDB can be modeled using two different approaches: embedded relationships or referenced relationships. The choice of these approaches will depend on the type of case to be addressed and data modeling decisions.

Below is a brief description of each approach:

Embedded relationship : A relationship that involves storing child documents embedded within a parent document.

Referenced relationship : Relationship that involves connecting different collections by means of references. The reference is made through a common property or key. These references can be of two types; manual references or by DBRefs .

NOTE 🧐: In this article we will rely on relationships with the referenced and manual type approach, using aggregation operators for the union.

Manual type references

When people talk about “manual references” in MongoDB, they usually refer to the practice of manually storing a field _idfrom a related document as a reference in another document.

This means that the developer is responsible for maintaining consistency of references and making the necessary queries to obtain full details of the related documents.

Cardinalities in manual references

With manual references in MongoDB, it is not necessary to explicitly express cardinalities in the reference definition. Cardinalities refer to the relationship between the number of documents in one collection relative to the number of documents in another collection.

This means that you can establish a one-to-one (1:1), one-to-many (1:N), or many-to-many (N:M) relationship using manual references, but the expression of cardinality is not done at the level of the reference itself.

Lookup Operator

One of the tools that MongoDB provides us to create relationships between documents is the operator $lookup . This operator aims to merge information from one document to another, using the left outer join method.

Finally, it is worth mentioning that the operator $lookupis used as a pipeline within the method aggregate() .

$lookup Syntax

{   
  $lookup: {       
    from: ,     
    localField: ,       
    foreignField: ,
    as:      
  }
}

Example of relationship Author(1) and books(N)

Below is a small example of a reference with a manual relationship between an author entity and a books entity, which represents the logic that an author can write many books.

db.createCollection(\"authors\");
db.createCollection(\"books\");

// Primera instrucción
db.authors.insertOne({
  name: \"Diego\",
  email: \"dcortes@example.com\",
  age: 25
});

// Segunda instrucción (Se debe obtener el ID del autor y remplazar)
db.books.insertMany([
  {
    name: \"Philosopher's Stone\",
    author_id: ObjectId(\"id_of_author\")
  },
  {
    name: \"Secret of programming\",
    author_id: ObjectId(\"id_of_author\")
  }
]);

Code explanation:

Two collections are being created in the database: authorsand books. These collections are used to store documents related to authors and books.
A document is being inserted into the collection authorswith an author's information. This author has a name ( Diego), an email ( dcortes@example.com) and an age (25).
Several documents are being inserted into the collection books. Each document represents a book and has a name and a field author_idthat should contain the ID of the book's author. In this case, the value of has been used ObjectId(\"id_of_author\").
This value should be replaced with the actual author ID that was inserted into the collection authorsin the first statement.

Querying information using aggregate and $lookup

Now, with our relationship and data prepared, we will proceed to perform the query based on the author and books logic.

db.getCollection(\"authors\").aggregate([{
  $lookup: {
    from: \"books\",
    localField: \"_id\",
    foreignField: \"author_id\",
    as: \"books\"
  }
}]);

//Resultado
{
    \"_id\" : ObjectId(\"64a8397a001cd56690c6a9cd\"),
    \"name\" : \"Diego\",
    \"email\" : \"dcortes@example.com\",
    \"age\" : NumberInt(25),
    \"books\" : [
        {
            \"_id\" : ObjectId(\"64a839a7001cd56690c6a9ce\"),
            \"name\" : \"Philosopher's Stone\",
            \"author_id\" : ObjectId(\"64a8397a001cd56690c6a9cd\")
        },
        {
            \"_id\" : ObjectId(\"64a839a7001cd56690c6a9cf\"),
            \"name\" : \"Secret of programming\",
            \"author_id\" : ObjectId(\"64a8397a001cd56690c6a9cd\")
        }
    ]
}

Code explanation:

This code performs an aggregation operation in MongoDB using the method aggregateon the \"authors\" collection. The aggregation operation uses the operator $lookupto perform a \"left outer join\" with the \"books\" collection.

db.getCollection(\"authors\"): Access the \"authors\" collection in the current database.
.aggregate([...]): Starts an aggregation operation. Aggregation in MongoDB allows complex transformations to be performed on data before returning the results.
{ $lookup: { ... } }: Use the operator $lookupto perform the union operation with the \"books\" collection. This operator is used to perform union operations between collections.

Inside $lookup:

from: \"books\": Specifies the collection to join with, in this case, \"books\".
localField: \"_id\": Specifies the field in the \"authors\" collection to use as the join key. In this case, \"_id\" from the \"authors\" collection is used.
foreignField: \"author_id\": Specifies the field in the \"books\" collection to use as the join key. In this case, \"author_id\" from the \"books\" collection is used.
as: \"books\": Specifies the name of the new field that will contain the results of the join. In this case, a field called \"books\" is created that will contain the matching documents from the \"books\" collection.

Source code

GitHub - dcortesnet/MongoDB-fundamentals: Fundamentals of the MongoDB database. Repository with examples and basic exercises of operations, copy and security, aggregation, relations and others. This repository is part of a self-taught supplemental course to help students develop skills.

Relationship Resources in MongoDB

Data Modeling and Patterns in MongoDB : Structuring strategies, examples, and common patterns.
$lookup Operator : Definitions, Syntax, and Examples.

Thanks for making it this far, if you found this helpful, don’t forget to clap 👏 . Subscribe for more content 🔔.

If you need additional help, please contact me 🤠.

@diego.coder | Linktree

Thank you very much for reading, I appreciate your time.

$\"\"$

Relationships in MongoDB? Yes, it is possible! Quick Guide 🚀⚡👌 was originally published in Dev Genius on Medium, where people are continuing the conversation by highlighting and responding to this story.

","contentEncoded","

Basic article on relationships in MongoDB, approaches, types of relationships, examples and features.

Recommendations

Before you start studying relationships in MongoDB, it is recommended to have some prior knowledge before reading this article, so I invite you to review the following optional support links:

Agregaciones y tuberías en Mongo DB (Aggregation Operations)

Relationships in MongoDB

The biggest distinction is that MongoDB does not faithfully adhere to the ER (Entity Relationship) model and other qualities of relational databases.

Relationship Approaches in MongoDB

Below is a brief description of each approach:

Embedded relationship : A relationship that involves storing child documents embedded within a parent document.

Referenced relationship : Relationship that involves connecting different collections by means of references. The reference is made through a common property or key. These references can be of two types; manual references or by DBRefs .

NOTE 🧐: In this article we will rely on relationships with the referenced and manual type approach, using aggregation operators for the union.

Manual type references

When people talk about “manual references” in MongoDB, they usually refer to the practice of manually storing a field _idfrom a related document as a reference in another document.

This means that the developer is responsible for maintaining consistency of references and making the necessary queries to obtain full details of the related documents.

Cardinalities in manual references

Lookup Operator

Finally, it is worth mentioning that the operator $lookupis used as a pipeline within the method aggregate() .

$lookup Syntax

{   
  $lookup: {       
    from: ,     
    localField: ,       
    foreignField: ,
    as:      
  }
}

Example of relationship Author(1) and books(N)

Below is a small example of a reference with a manual relationship between an author entity and a books entity, which represents the logic that an author can write many books.

db.createCollection(\"authors\");
db.createCollection(\"books\");

// Primera instrucción
db.authors.insertOne({
  name: \"Diego\",
  email: \"dcortes@example.com\",
  age: 25
});

// Segunda instrucción (Se debe obtener el ID del autor y remplazar)
db.books.insertMany([
  {
    name: \"Philosopher's Stone\",
    author_id: ObjectId(\"id_of_author\")
  },
  {
    name: \"Secret of programming\",
    author_id: ObjectId(\"id_of_author\")
  }
]);

Code explanation:

Two collections are being created in the database: authorsand books. These collections are used to store documents related to authors and books.
A document is being inserted into the collection authorswith an author's information. This author has a name ( Diego), an email ( dcortes@example.com) and an age (25).
Several documents are being inserted into the collection books. Each document represents a book and has a name and a field author_idthat should contain the ID of the book's author. In this case, the value of has been used ObjectId(\"id_of_author\").
This value should be replaced with the actual author ID that was inserted into the collection authorsin the first statement.

Querying information using aggregate and $lookup

Now, with our relationship and data prepared, we will proceed to perform the query based on the author and books logic.

db.getCollection(\"authors\").aggregate([{
  $lookup: {
    from: \"books\",
    localField: \"_id\",
    foreignField: \"author_id\",
    as: \"books\"
  }
}]);

//Resultado
{
    \"_id\" : ObjectId(\"64a8397a001cd56690c6a9cd\"),
    \"name\" : \"Diego\",
    \"email\" : \"dcortes@example.com\",
    \"age\" : NumberInt(25),
    \"books\" : [
        {
            \"_id\" : ObjectId(\"64a839a7001cd56690c6a9ce\"),
            \"name\" : \"Philosopher's Stone\",
            \"author_id\" : ObjectId(\"64a8397a001cd56690c6a9cd\")
        },
        {
            \"_id\" : ObjectId(\"64a839a7001cd56690c6a9cf\"),
            \"name\" : \"Secret of programming\",
            \"author_id\" : ObjectId(\"64a8397a001cd56690c6a9cd\")
        }
    ]
}

Code explanation:

db.getCollection(\"authors\"): Access the \"authors\" collection in the current database.
.aggregate([...]): Starts an aggregation operation. Aggregation in MongoDB allows complex transformations to be performed on data before returning the results.
{ $lookup: { ... } }: Use the operator $lookupto perform the union operation with the \"books\" collection. This operator is used to perform union operations between collections.

Inside $lookup:

from: \"books\": Specifies the collection to join with, in this case, \"books\".
localField: \"_id\": Specifies the field in the \"authors\" collection to use as the join key. In this case, \"_id\" from the \"authors\" collection is used.
foreignField: \"author_id\": Specifies the field in the \"books\" collection to use as the join key. In this case, \"author_id\" from the \"books\" collection is used.
as: \"books\": Specifies the name of the new field that will contain the results of the join. In this case, a field called \"books\" is created that will contain the matching documents from the \"books\" collection.

Source code

Relationship Resources in MongoDB

Data Modeling and Patterns in MongoDB : Structuring strategies, examples, and common patterns.
$lookup Operator : Definitions, Syntax, and Examples.

Thanks for making it this far, if you found this helpful, don’t forget to clap 👏 . Subscribe for more content 🔔.

If you need additional help, please contact me 🤠.

@diego.coder | Linktree

Thank you very much for reading, I appreciate your time.

$\"\"$

","freeContentEncoded","textContent","Basic article on relationships in MongoDB, approaches, types of relationships, examples and features.RecommendationsBefore you start studying relationships in MongoDB, it is recommended to have some prior knowledge before reading this article, so I invite you to review the following optional support links:Agregaciones y tuberías en Mongo DB (Aggregation Operations)Relationships in MongoDBOne of the criteria that tends to be confused is the term NoSQL databases with the absence of relationships, but the truth is that many of the NoSQL databases already offer us functionalities that allow us to have a certain degree of relationship between the data.For a couple of years now, MongoDB has allowed the implementation of relationships through a concept called “ references ”. It is worth noting that even the official MongoDB documentation talks about cardinalities and other characteristics that are known in a relational database.The biggest distinction is that MongoDB does not faithfully adhere to the ER (Entity Relationship) model and other qualities of relational databases.Relationship Approaches in MongoDBRelationships in MongoDB can be modeled using two different approaches: embedded relationships or referenced relationships. The choice of these approaches will depend on the type of case to be addressed and data modeling decisions.Below is a brief description of each approach:Embedded relationship : A relationship that involves storing child documents embedded within a parent document.Referenced relationship : Relationship that involves connecting different collections by means of references. The reference is made through a common property or key. These references can be of two types; manual references or by DBRefs .NOTE 🧐: In this article we will rely on relationships with the referenced and manual type approach, using aggregation operators for the union.Manual type referencesWhen people talk about “manual references” in MongoDB, they usually refer to the practice of manually storing a field _idfrom a related document as a reference in another document.This means that the developer is responsible for maintaining consistency of references and making the necessary queries to obtain full details of the related documents.Cardinalities in manual referencesWith manual references in MongoDB, it is not necessary to explicitly express cardinalities in the reference definition. Cardinalities refer to the relationship between the number of documents in one collection relative to the number of documents in another collection.This means that you can establish a one-to-one (1:1), one-to-many (1:N), or many-to-many (N:M) relationship using manual references, but the expression of cardinality is not done at the level of the reference itself.Lookup OperatorOne of the tools that MongoDB provides us to create relationships between documents is the operator $lookup . This operator aims to merge information from one document to another, using the left outer join method.Finally, it is worth mentioning that the operator $lookupis used as a pipeline within the method aggregate() .$lookup Syntax{ $lookup: { from: , localField: , foreignField: , as: }}Example of relationship Author(1) and books(N)Below is a small example of a reference with a manual relationship between an author entity and a books entity, which represents the logic that an author can write many books.db.createCollection(\"authors\");db.createCollection(\"books\");// Primera instruccióndb.authors.insertOne({ name: \"Diego\", email: \"dcortes@example.com\", age: 25});// Segunda instrucción (Se debe obtener el ID del autor y remplazar)db.books.insertMany([ { name: \"Philosopher's Stone\", author_id: ObjectId(\"id_of_author\") }, { name: \"Secret of programming\", author_id: ObjectId(\"id_of_author\") }]);Code explanation:Two collections are being created in the database: authorsand books. These collections are used to store documents related to authors and books.A document is being inserted into the collection authorswith an author's information. This author has a name ( Diego), an email ( dcortes@example.com) and an age (25).Several documents are being inserted into the collection books. Each document represents a book and has a name and a field author_idthat should contain the ID of the book's author. In this case, the value of has been used ObjectId(\"id_of_author\").This value should be replaced with the actual author ID that was inserted into the collection authorsin the first statement.Querying information using aggregate and $lookupNow, with our relationship and data prepared, we will proceed to perform the query based on the author and books logic.db.getCollection(\"authors\").aggregate([{ $lookup: { from: \"books\", localField: \"_id\", foreignField: \"author_id\", as: \"books\" }}]);//Resultado{ \"_id\" : ObjectId(\"64a8397a001cd56690c6a9cd\"), \"name\" : \"Diego\", \"email\" : \"dcortes@example.com\", \"age\" : NumberInt(25), \"books\" : [ { \"_id\" : ObjectId(\"64a839a7001cd56690c6a9ce\"), \"name\" : \"Philosopher's Stone\", \"author_id\" : ObjectId(\"64a8397a001cd56690c6a9cd\") }, { \"_id\" : ObjectId(\"64a839a7001cd56690c6a9cf\"), \"name\" : \"Secret of programming\", \"author_id\" : ObjectId(\"64a8397a001cd56690c6a9cd\") } ]}Code explanation:This code performs an aggregation operation in MongoDB using the method aggregateon the \"authors\" collection. The aggregation operation uses the operator $lookupto perform a \"left outer join\" with the \"books\" collection.db.getCollection(\"authors\"): Access the \"authors\" collection in the current database..aggregate([...]): Starts an aggregation operation. Aggregation in MongoDB allows complex transformations to be performed on data before returning the results.{ $lookup: { ... } }: Use the operator $lookupto perform the union operation with the \"books\" collection. This operator is used to perform union operations between collections.Inside $lookup:from: \"books\": Specifies the collection to join with, in this case, \"books\".localField: \"_id\": Specifies the field in the \"authors\" collection to use as the join key. In this case, \"_id\" from the \"authors\" collection is used.foreignField: \"author_id\": Specifies the field in the \"books\" collection to use as the join key. In this case, \"author_id\" from the \"books\" collection is used.as: \"books\": Specifies the name of the new field that will contain the results of the join. In this case, a field called \"books\" is created that will contain the matching documents from the \"books\" collection.Source codeGitHub - dcortesnet/MongoDB-fundamentals: Fundamentals of the MongoDB database. Repository with examples and basic exercises of operations, copy and security, aggregation, relations and others. This repository is part of a self-taught supplemental course to help students develop skills.Relationship Resources in MongoDBData Modeling and Patterns in MongoDB : Structuring strategies, examples, and common patterns.$lookup Operator : Definitions, Syntax, and Examples.Thanks for making it this far, if you found this helpful, don’t forget to clap 👏 . Subscribe for more content 🔔.If you need additional help, please contact me 🤠.@diego.coder | LinktreeThank you very much for reading, I appreciate your time.Relationships in MongoDB? Yes, it is possible! 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