Schema Validation in MongoDB allows you to define rules for the structure of documents within a collection. By specifying a schema, you can enforce data consistency and ensure that documents conform to certain constraints. Here’s how you can define schema validation for a MongoDB collection with the keys "title", "body", "category", "links", "tags", and "date".

Example Schema Validation

Suppose you have a collection named posts, and you want to enforce validation rules on the following fields:

• title: A string that must be present.
• body: A string that must be present.
• category: A string that must be present and limited to a set of predefined values.
• links: An array of strings where each string must be a valid URL.
• tags: An array of strings where each string is a tag.
• date: A date field that must be a valid ISO date format.

Here’s how you can define schema validation for this structure using MongoDB's JSON Schema validator:

Defining Validation Rules

1. Create the Collection with Validation Rules: You can create the collection with schema validation rules using the createCollection command.

   db.createCollection("posts", {
     validator: {
       $jsonSchema: {
         bsonType: "object",
         required: ["title", "body", "category", "date"], // Fields that must be present
         properties: {
           title: {
             bsonType: "string",
             description: "Title must be a string and is required"
           },
           body: {
             bsonType: "string",
             description: "Body must be a string and is required"
           },
           category: {
             bsonType: "string",
             enum: ["news", "blog", "article"], // Restrict to these predefined values
             description: "Category must be one of the specified values and is required"
           },
           links: {
             bsonType: "array",
             items: {
               bsonType: "string",
               pattern: "^(http|https)://.*$" // Must be a valid URL
             },
             description: "Links must be an array of valid URLs"
           },
           tags: {
             bsonType: "array",
             items: {
               bsonType: "string"
             },
             description: "Tags must be an array of strings"
           },
           date: {
             bsonType: "date",
             description: "Date must be a valid ISODate"
           }
         }
       }
     },
     validationAction: "error", // Use "warn" if you want to log warnings instead of errors
     validationLevel: "strict" // Use "moderate" to allow existing documents not matching the schema
   });
   

2. Update Validation Rules for an Existing Collection: If you need to modify the validation rules for an existing collection, you can use the collMod command.

   db.runCommand({
     collMod: "posts",
     validator: {
       $jsonSchema: {
         bsonType: "object",
         required: ["title", "body", "category", "date"], // Ensure fields are present
         properties: {
           title: {
             bsonType: "string",
             description: "Title must be a string and is required"
           },
           body: {
             bsonType: "string",
             description: "Body must be a string and is required"
           },
           category: {
             bsonType: "string",
             enum: ["news", "blog", "article"], // Enforce predefined values
             description: "Category must be one of the specified values and is required"
           },
           links: {
             bsonType: "array",
             items: {
               bsonType: "string",
               pattern: "^(http|https)://.*$" // Validate URLs
             },
             description: "Links must be an array of valid URLs"
           },
           tags: {
             bsonType: "array",
             items: {
               bsonType: "string"
             },
             description: "Tags must be an array of strings"
           },
           date: {
             bsonType: "date",
             description: "Date must be a valid ISODate"
           }
         }
       }
     },
     validationAction: "error", // Set to "warn" for warnings
     validationLevel: "strict" // Change to "moderate" if needed
   });
   

Key Points of Schema Validation

• Required Fields: The required array specifies which fields must be present in the document.
• Data Types: bsonType specifies the expected data type for each field (e.g., string, date, array).
• Enumerations: The enum field restricts the value to a predefined set of options (useful for fields like category).
• Pattern Matching: The pattern field is used to validate string formats (e.g., ensuring URLs are valid).
• Array Validation: You can validate items within an array, such as ensuring that links contains only valid URLs.
• Validation Actions: validationAction can be set to error to reject invalid documents or warn to log warnings without rejecting them.
• Validation Levels: validationLevel can be strict (enforces schema for all documents) or moderate (enforces schema only for new or modified documents).