The development and use of plugins

Basic Concepts of Plugins

Mongoose plugins are reusable functional modules that can add additional functionality or modify existing behavior to a Schema. By encapsulating common logic, plugins avoid code duplication, making Schema functionality extensions more modular and maintainable. Each plugin is essentially a function that takes a schema and options as parameters and can add methods, static methods, virtual properties, middleware, etc., to the schema.

// A simple plugin example
function timestampPlugin(schema, options) {
  schema.add({ 
    createdAt: Date,
    updatedAt: Date 
  });

  schema.pre('save', function(next) {
    const now = new Date();
    this.updatedAt = now;
    if (!this.createdAt) {
      this.createdAt = now;
    }
    next();
  });
}

Plugin Development Methods

Developing a Mongoose plugin requires following a specific pattern. First, define a function that takes a schema and optional options parameters. Within the function body, you can perform various operations on the schema:

1. Add Fields: Use the schema.add() method to add new fields.
2. Define Methods: Add instance methods via schema.methods.
3. Define Static Methods: Add static methods via schema.statics.
4. Add Middleware: Use pre and post hooks.
5. Define Virtual Properties: Use schema.virtual().

function paginatePlugin(schema, options) {
  const defaultOptions = {
    perPage: 10,
    maxPerPage: 50
  };
  const opts = { ...defaultOptions, ...options };

  // Add static method
  schema.statics.paginate = async function(query, page = 1, perPage = opts.perPage) {
    const skip = (page - 1) * perPage;
    const limit = Math.min(perPage, opts.maxPerPage);
    const [total, items] = await Promise.all([
      this.countDocuments(query),
      this.find(query).skip(skip).limit(limit)
    ]);
    return {
      total,
      page,
      perPage: limit,
      totalPages: Math.ceil(total / limit),
      items
    };
  };
}

How to Use Plugins

Using plugins is straightforward. Simply call the plugin() method after defining the schema. You can pass options to customize the plugin's behavior.

const mongoose = require('mongoose');
const { Schema } = mongoose;

const userSchema = new Schema({
  name: String,
  email: String
});

// Apply plugins
userSchema.plugin(timestampPlugin);
userSchema.plugin(paginatePlugin, { maxPerPage: 30 });

const User = mongoose.model('User', userSchema);

// Use functionality added by plugins
async function getUsers(page) {
  return await User.paginate({}, page);
}

Common Plugin Development Patterns

1. Audit Log Plugin

Records document creation and modification times, along with user information.

function auditLogPlugin(schema, options) {
  const userModel = options.userModel || 'User';
  
  schema.add({
    createdBy: { type: Schema.Types.ObjectId, ref: userModel },
    updatedBy: { type: Schema.Types.ObjectId, ref: userModel },
    createdAt: Date,
    updatedAt: Date
  });

  schema.pre('save', function(next) {
    const now = new Date();
    this.updatedAt = now;
    if (this.isNew) {
      this.createdAt = now;
      // Assume context contains current user info
      if (this.$context && this.$context.userId) {
        this.createdBy = this.$context.userId;
      }
    }
    if (this.isModified() && this.$context && this.$context.userId) {
      this.updatedBy = this.$context.userId;
    }
    next();
  });
}

2. Soft Delete Plugin

Implements soft deletion instead of physically removing documents from the database.

function softDeletePlugin(schema, options) {
  const deletedAtField = options.deletedAtField || 'deletedAt';
  const isDeletedField = options.isDeletedField || 'isDeleted';
  
  schema.add({
    [deletedAtField]: Date,
    [isDeletedField]: { type: Boolean, default: false }
  });

  // Add delete method
  schema.methods.softDelete = function() {
    this[isDeletedField] = true;
    this[deletedAtField] = new Date();
    return this.save();
  };

  // Add restore method
  schema.methods.restore = function() {
    this[isDeletedField] = false;
    this[deletedAtField] = undefined;
    return this.save();
  };

  // Modify query behavior to automatically filter deleted documents
  schema.pre(/^find/, function() {
    if (!this.getFilter()[isDeletedField]) {
      this.where({ [isDeletedField]: false });
    }
  });
}

Advanced Plugin Techniques

1. Plugin Composition

Multiple plugins can be combined, but be mindful of execution order and potential conflicts.

const productSchema = new Schema({
  name: String,
  price: Number
});

productSchema.plugin(timestampPlugin);
productSchema.plugin(auditLogPlugin, { userModel: 'Admin' });
productSchema.plugin(softDeletePlugin);

2. Conditional Plugin Application

Plugins can be applied based on environment or other conditions.

if (process.env.NODE_ENV === 'development') {
  const debugPlugin = require('./debug-plugin');
  userSchema.plugin(debugPlugin);
}

3. Plugin Dependency Management

When plugins have dependencies, ensure they are loaded in the correct order.

function dependentPlugin(schema, options) {
  // Check if the base plugin is installed
  if (!schema.methods.baseMethod) {
    throw new Error('dependentPlugin requires basePlugin to be installed first');
  }
  // Plugin implementation...
}

Practical Use Cases

1. Multilingual Support Plugin

Adds multilingual field support to documents.

function multilingualPlugin(schema, options) {
  const { fields = [], languages = ['en', 'zh'] } = options;

  fields.forEach(field => {
    const multilingualField = {};
    languages.forEach(lang => {
      multilingualField[lang] = schema.path(field).instance;
    });
    schema.add({ [field]: multilingualField });
    schema.remove(field);

    // Add method to get value in current language
    schema.method(`get${field.charAt(0).toUpperCase() + field.slice(1)}`, function(lang) {
      return this[field][lang] || this[field][languages[0]];
    });
  });
}

// Usage example
const productSchema = new Schema({
  name: String,
  description: String
});

productSchema.plugin(multilingualPlugin, {
  fields: ['name', 'description'],
  languages: ['en', 'zh', 'ja']
});

2. Version Control Plugin

Implements document versioning.

function versionControlPlugin(schema, options) {
  const versionSchema = new Schema({
    refId: Schema.Types.ObjectId,
    data: {},
    version: Number,
    modifiedBy: Schema.Types.ObjectId,
    modifiedAt: Date
  }, { timestamps: true });

  const VersionModel = mongoose.model(options.versionModel || 'Version', versionSchema);

  schema.pre('save', async function(next) {
    if (this.isModified()) {
      const versionData = this.toObject();
      delete versionData._id;
      delete versionData.__v;
      
      await VersionModel.create({
        refId: this._id,
        data: versionData,
        version: (await VersionModel.countDocuments({ refId: this._id })) + 1,
        modifiedBy: this.$context?.userId
      });
    }
    next();
  });

  schema.methods.getVersions = function() {
    return VersionModel.find({ refId: this._id }).sort('-version');
  };

  schema.statics.restoreVersion = async function(id, version) {
    const doc = await this.findById(id);
    const versionDoc = await VersionModel.findOne({ refId: id, version });
    if (!versionDoc) throw new Error('Version not found');
    
    doc.set(versionDoc.data);
    return doc.save();
  };
}

Performance Optimization Considerations

When developing plugins, consider performance impacts:

1. Avoid Unnecessary Middleware: Only add pre/post hooks when needed.
2. Batch Operation Optimization: Consider scenarios like updateMany.
3. Index Management: Fields added by plugins may require indexing.
4. Query Optimization: Avoid degrading query performance.

function optimizedPlugin(schema) {
  // Only add indexes when necessary
  if (process.env.NODE_ENV === 'production') {
    schema.index({ updatedAt: -1 });
  }

  // Optimize bulk updates
  schema.pre('updateMany', function(next) {
    this.update({}, { $set: { updatedAt: new Date() } });
    next();
  });
}

Best Practices for Testing Plugins

Writing tests for plugins is crucial for ensuring reliability:

1. Isolated Testing: Test plugin functionality separately.
2. Multiple Scenarios: Test with different option configurations.
3. Performance Testing: Ensure no performance issues are introduced.
4. Compatibility Testing: Test compatibility with other commonly used plugins.

describe('timestampPlugin', () => {
  let TestModel;
  
  before(() => {
    const schema = new Schema({ name: String });
    schema.plugin(timestampPlugin);
    TestModel = mongoose.model('Test', schema);
  });

  it('should add createdAt and updatedAt fields', async () => {
    const doc = await TestModel.create({ name: 'test' });
    expect(doc.createdAt).to.be.instanceOf(Date);
    expect(doc.updatedAt).to.be.instanceOf(Date);
  });

  it('should update updatedAt on save', async () => {
    const doc = await TestModel.create({ name: 'test' });
    const originalUpdatedAt = doc.updatedAt;
    await new Promise(resolve => setTimeout(resolve, 10));
    doc.name = 'updated';
    await doc.save();
    expect(doc.updatedAt.getTime()).to.be.greaterThan(originalUpdatedAt.getTime());
  });
});

Plugin Ecosystem

Mongoose has a rich plugin ecosystem. Some popular plugins include:

1. mongoose-autopopulate: Auto-populates referenced fields.
2. mongoose-paginate-v2: Pagination functionality.
3. mongoose-unique-validator: Uniqueness validation.
4. mongoose-lean-virtuals: Includes virtual fields in lean queries.

// Example of using a third-party plugin
const uniqueValidator = require('mongoose-unique-validator');

const userSchema = new Schema({
  email: { type: String, required: true, unique: true }
});

userSchema.plugin(uniqueValidator, { 
  message: 'Error, expected {PATH} to be unique.' 
});

Advanced Patterns for Custom Plugins

For more complex scenarios, consider these advanced patterns:

1. Dynamic Field Plugin: Adds fields dynamically based on configuration.
2. State Machine Plugin: Implements document state transitions.
3. Permission Control Plugin: Role-based field-level access control.
4. Caching Plugin: Automatically caches query results.

function stateMachinePlugin(schema, options) {
  const { field = 'status', states, transitions } = options;
  
  schema.add({ [field]: { type: String, default: states.initial } });

  schema.methods.canTransitionTo = function(newState) {
    return transitions[this[field]].includes(newState);
  };

  schema.methods.transitionTo = function(newState) {
    if (!this.canTransitionTo(newState)) {
      throw new Error(`Invalid transition from ${this[field]} to ${newState}`);
    }
    this[field] = newState;
    return this.save();
  };

  // Add state-checking methods
  states.valid.forEach(state => {
    schema.methods[`is${state.charAt(0).toUpperCase() + state.slice(1)}`] = function() {
      return this[field] === state;
    };
  });
}

// Usage example
const taskSchema = new Schema({
  title: String
});

taskSchema.plugin(stateMachinePlugin, {
  states: {
    initial: 'new',
    valid: ['new', 'inProgress', 'completed', 'archived']
  },
  transitions: {
    new: ['inProgress'],
    inProgress: ['completed', 'new'],
    completed: ['archived'],
    archived: []
  }
});