Mongodb多对多关系

Question

I am working on a webapp built on mean.js and part of the app is projects that have tags, similar to a blog site that has tags for each blog entry.

I have searched everywhere for a good example/tutorial explaining the best way to create a many to many relationship using mongodb/mongoose, but I can't seem to find anything.

Each project can have multiple tags and I want the users to be able to find all projects with a specific tag.

I would really appreciate any suggestions/examples on the correct way to achieve this. TIA

Answer 1

Keep an array of id's in both collections. For example:

coll1:

{
  _id: ObjectId("56784ac717e12e59d600688a"),
  coll2_ids: [ObjectId("..."), ObjectId("..."), ObjectId("..."), ...]
}

coll2:

{
  _id: ObjectId("56784ac717e12e59d600677b"),
  coll1_ids: [ObjectId("..."), ObjectId("..."), ObjectId("..."), ...]
}

The advantage of this approach is that when you have a document from either collection, you can immediately pull all associated documents from the opposite collection simply by doing

obj = db.coll1.findOne({})
db.coll2.find({_id: {$in: obj['coll2_ids']}}) # returns all associated documents

and vice-versa.

Answer 2

For many-to-many relationship, I have found mongoose-relationship really useful.

Here's a sample how you would solve it using mongoose-relationship module:

// In TagsModel.js file ----------------------------------------------
var mongoose = require('mongoose');
var schema = mongoose.Schema;
var relationship = require("mongoose-relationship");

var tagSchema = new schema({
   projects: [{
      type: mongoose.Schema.Types.ObjectId,
      ref: 'Project',
      childPath: 'tags'
   }],
   ....
});

tagSchema.plugin(relationship, {
     relationshipPathName: 'projects'
});
var tagModel = mongoose.model('Tag', tagSchema);
--------------------------------------------------------------------

// In ProjectModel.js file -----------------------------------------
var mongoose = require('mongoose');
var schema = mongoose.Schema;

var Projects = new Schema({
    tags : [{
         type: mongoose.Schema.Types.ObjectId,
         ref: 'Tag'
         }],
    ...
});
var tagModel = mongoose.model('Project', tagSchema);

With this model structure, you will be able to find projects by specific tag and tags by project.

Answer 3

It seems you just want to have an Array of tags within your Project schema.

var Tags = new Schema({
    name : String
});

var Projects = new Schema({
    tags : [String] // When you save a Tag just save the Name of it here
});

// This way you could easily look it up
// Assuming "name" was passed in as a param to express

Projects.find({ 'tags' : req.params.name }) // voila!

There are other ways as well (such as saving ObjectIds ). This method would be easier if you think Tag names could change often but once again if a Tag is "deleted" you'd have to go look through all Projects to remove that ObjectId

tags : [{ type: Schema.ObjectId, ref : 'Tags' }]

Basically the gist of this is that you are saving the a String or ObjectId reference (of the name) in an Array of "tags" within your Project model.

Just remember when you're going to Edit Tag names / Delete a tag / etc, you'll want to go through and update (if they are saved as Strings) / remove those tags from any Projects that have it in their array.