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Mongoose allows documents to reference other documents. Referencing documents is known as normalization. It is good for consistency but creates more queries in the system.

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Below, we have two models: Author & Course, and we are referencing an author document from the course schema.

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models/authorModel.js
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const mongoose = require("mongoose");\n\nconst authorSchema = new mongoose.Schema({\n   name: String,\n   profession: String,\n});\n\nconst Author = mongoose.model("Author", authorSchema);\n\nmodule.exports = Author;
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models/courseModel.js
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const mongoose = require("mongoose");\n\nconst Author = require("./authorModel");\n\nconst courseSchema = new mongoose.Schema({\n    name: String,\n    price: Number,\n    author: {\n      type: mongoose.Schema.Types.ObjectId,\n      ref: "Author",\n    },\n});\n\nconst Course = mongoose.model("Course", courseSchema);\n\nmodule.exports = Course;
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The value of the ref property is usually the name of the model mongoose should use to load referenced documents.

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We create an author document.

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index.js
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const Author = require("./models/authorModel");\nconst Course = require("./models/courseModel");\n\n\nconst createAuthor = async (name, profession) => {\n    const author = new Author({\n       name,\n       profession,\n    });\n\n    const result = await author.save();\n\n    console.log(result)\n};\n\ncreateAuthor("Hemanta", "Full-stack Developer");
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{\n   _id: 60b606c4f2e9693af47f62c8,\n   name: 'Hemanta',\n   profession: 'Full-stack Developer',\n   __v: 0\n}
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Now, we create a course document.

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index.js
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const createCourse = async (name, price, author) => {\n  const course = new Course({\n    name,\n    price,\n    author,\n  })\n\n  const result = await course.save()\n  console.log(result)\n}\n\ncreateCourse("Master Node", 19.99, "60b606c4f2e9693af47f62c8")
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{\n   _id: 60b6075e9941a80730a431f1,\n   name: 'Master Node',\n   price: 19.99,\n   author: 60b606c4f2e9693af47f62c8,\n   __v: 0\n}
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Now we fetch the course we created.

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const listCourses = async () => {\n  const courses = await Course.find()\n  console.log(courses)\n}\n\nlistCourses()
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{\n  _id: 60b6075e9941a80730a431f1,\n  name: 'Master Node',\n  price: 19.99,\n  author: 60b606c4f2e9693af47f62c8,\n  __v: 0\n}
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We only see the author Id. If we want to see all the properties of the author document, we can use the populate method.

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Population

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Population is the process of automatically replacing the specified paths in the document with documents from other collections. We can load referenced documents using the populate() method.

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index.js
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const listCourses = async () => {\n  const courses = await Course.find().populate("author")\n  console.log(courses)\n}\n\nlistCourses()
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Including property

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The author property of a course object can contain many properties. So, we can choose to include or exclude certain properties by passing a second argument to the populate method.

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const listCourses = async () => {\n  const courses = await Course.find().populate("author", "name")\n  console.log(courses)\n}\n\nlistCourses()
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Excluding property

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Similarly, we can choose to exclude properties of the referenced document using a dash(-). Below, we have included the name property and excluded the _id property of the author document.

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const listCourses = async () => {\n  const courses = await Course.find().populate("author", "name -_id")\n  console.log(courses)\n}\n\nlistCourses()
\n","frontmatter":{"title":"Modelling Relationships in Mongoose Using Reference","date":"2021-06-01"}}},"pageContext":{"slug":"/modelling-relationships-in-mongoose-using-reference/","prev":{"fields":{"slug":"/how-to-set-up-templating-engines-with-express/"},"frontmatter":{"modules":null}},"next":{"fields":{"slug":"/validation-in-mongoose/"},"frontmatter":{"modules":null}}}}, "staticQueryHashes": ["3159585216"]}