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What's Bongo?

We couldn't find a good ODM for MongoDB written in Go, so we made one. Bongo is a wrapper for mgo ( that adds ODM, hooks, validation, and cascade support to its raw Mongo functions.

Bongo is tested using the fantasic GoConvey (

Build Status

Coverage Status


Since we're not yet at a major release, some things in the API might change. Here's a list:

  • Save - stable
  • Find/FindOne/FindById - stable
  • Delete - stable
  • Save/Delete/Find/Validation hooks - stable
  • Cascade - unstable (might need a refactor)
  • Change Tracking - stable
  • Validation methods - stable


Basic Usage

Import the Library

go get

import ""

And install dependencies:

cd $GOHOME/src/ && go get .

Connect to a Database

Create a new bongo.Config instance:

config := &bongo.Config{
	ConnectionString: "localhost",
	Database:         "bongotest",

Then just create a new instance of bongo.Connection, and make sure to handle any connection errors:

connection, err := bongo.Connect(config)

if err != nil {

If you need to, you can access the raw mgo session with connection.Session

Create a Document

Any struct can be used as a document as long as it satisfies the Document interface (SetId(bson.ObjectId), GetId() bson.ObjectId). We recommend that you use the DocumentBase provided with Bongo, which implements that interface as well as the NewTracker, TimeCreatedTracker and TimeModifiedTracker interfaces (to keep track of new/existing documents and created/modified timestamps). If you use the DocumentBase or something similar, make sure you use bson:",inline" otherwise you will get nested behavior when the data goes to your database.

For example:

type Person struct {
	bongo.DocumentBase `bson:",inline"`
	FirstName string
	LastName string
	Gender string

You can use child structs as well.

type Person struct {
	bongo.DocumentBase `bson:",inline"`
	FirstName string
	LastName string
	Gender string
	HomeAddress struct {
		Street string
		Suite string
		City string
		State string
		Zip string


You can add special methods to your document type that will automatically get called by bongo during certain actions. Hooks get passed the current *bongo.Collection so you can avoid having to couple them with your actual database layer. Currently available hooks are:

  • func (s *ModelStruct) Validate(*bongo.Collection) []error (returns a slice of errors - if it is empty then it is assumed that validation succeeded)
  • func (s *ModelStruct) BeforeSave(*bongo.Collection) error
  • func (s *ModelStruct) AfterSave(*bongo.Collection) error
  • func (s *ModelStruct) BeforeDelete(*bongo.Collection) error
  • func (s *ModelStruct) AfterDelete(*bongo.Collection) error
  • func (s *ModelStruct) AfterFind(*bongo.Collection) error

Saving Models

Just call save on a collection instance.

myPerson := &Person{
err := connection.Collection("people").Save(myPerson)

Now you'll have a new document in the people collection. If there is an error, you can check if it is a validation error using a type assertion:

if vErr, ok := err.(*bongo.ValidationError); ok {
	fmt.Println("Validation errors are:", vErr.Errors)
} else {
	fmt.Println("Got a real error:", err.Error())

Deleting Documents

There are three ways to delete a document.


Same thing as Save - just call DeleteDocument on the collection and pass the document instance.

err := connection.Collection("people").DeleteDocument(person)

This will run the BeforeDelete and AfterDelete hooks, if applicable.


This just delegates to mgo.Collection.Remove. It will not run the BeforeDelete and AfterDelete hooks.

err := connection.Collection("people").DeleteOne(bson.M{"FirstName":"Testy"})


This delegates to mgo.Collection.RemoveAll. It will not run the BeforeDelete and AfterDelete hooks.

changeInfo, err := connection.Collection("people").Delete(bson.M{"FirstName":"Testy"})
fmt.Printf("Deleted %d documents", changeInfo.Removed)

Find by ID

person := &Person{}
err := connection.Collection("people").FindById(bson.ObjectIdHex(StringId), person)

The error returned can be a DocumentNotFoundError or a more low-level MongoDB error. To check, use a type assertion:

if dnfError, ok := err.(*bongo.DocumentNotFoundError); ok {
	fmt.Println("document not found")
} else {
	fmt.Println("real error " + err.Error())


Finds will return an instance of ResultSet, which you can then optionally Paginate and iterate through to get all results.

// *bongo.ResultSet
results := connection.Collection("people").Find(bson.M{"firstName":"Bob"})

person := &Person{}

count := 0

for results.Next(person) {

To paginate, you can run Paginate(perPage int, currentPage int) on the result of connection.Find(). That will return an instance of bongo.PaginationInfo, with properties like TotalRecords, RecordsOnPage, etc.

To use additional functions like sort, skip, limit, etc, you can access the underlying mgo Query via ResultSet.Query.

Find One

Same as find, but it will populate the reference of the struct you provide as the second argument.

person := &Person{}

err := connection.Collection("people").FindOne(bson.M{"firstName":"Bob"}, person)

if err != nil {
} else {
	fmt.Println("Found user:", person.FirstName)

Change Tracking

If your model struct implements the Trackable interface, it will automatically track changes to your model so you can compare the current values with the original. For example:

type MyModel struct {
	bongo.DocumentBase `bson:",inline"`
	StringVal string
	diffTracker *bongo.DiffTracker

// Easy way to lazy load a diff tracker
func (m *MyModel) GetDiffTracker() *DiffTracker {
	if m.diffTracker == nil {
		m.diffTracker = bongo.NewDiffTracker(m)

	return m.diffTracker

myModel := &MyModel{}

Use as follows:

Check if a field has been modified

// Store the current state for comparison

// Change a property...
myModel.StringVal = "foo"

// We know it's been instantiated so no need to use GetDiffTracker()
fmt.Println(myModel.diffTracker.Modified("StringVal")) // true
fmt.Println(myModel.diffTracker.Modified("StringVal")) // false

Get all modified fields

myModel.StringVal = "foo"
// Store the current state for comparison

isNew, modifiedFields := myModel.GetModified()

fmt.Println(isNew, modifiedFields) // false, ["StringVal"]

isNew, modifiedFields = myModel.GetModified()
fmt.Println(isNew, modifiedFields) // false, []

Diff-tracking Session

If you are going to be checking more than one field, you should instantiate a new DiffTrackingSession with diffTracker.NewSession(useBsonTags bool). This will load the changed fields into the session. Otherwise with each call to diffTracker.Modified(), it will have to recalculate the changed fields.

Cascade Save/Delete

Bongo supports cascading portions of documents to related documents and the subsequent cleanup upon deletion. For example, if you have a Team collection, and each team has an array of Players, you can cascade a player's first name and last name to his or her team.Players array on save, and remove that element in the array if you delete the player.

To use this feature, your struct needs to have an exported method called GetCascade, which returns an array of *bongo.CascadeConfig. Additionally, if you want to make use of the OldQuery property to remove references from previously related documents, you should probably alsotimplement the DiffTracker on your model struct (see above).

You can also leave ThroughProp blank, in which case the properties of the document will be cascaded directly onto the related document. This is useful when you want to cascade ObjectId properties or other references, but it is important that you keep in mind that these properties will be nullified on the related document when the main doc is deleted or changes references.

Also note that like the above hooks, the GetCascade method will be passed the instance of the bongo.Collection so you can keep your models decoupled from your database layer.

Casade Configuration

type CascadeConfig struct {
	// The collection to cascade to
	Collection *mgo.Collection

	// The relation type (does the target doc have an array of these docs [REL_MANY] or just reference a single doc [REL_ONE])
	RelType int

	// The property on the related doc to populate
	ThroughProp string

	// The query to find related docs
	Query bson.M

	// The data that constructs the query may have changed - this is to remove self from previous relations
	OldQuery bson.M

	// Properties that will be cascaded/deleted. Can (should) be in dot notation for nested properties. This is used to nullify properties when there is an OldQuery or if the document is deleted.
	Properties []string

	// The actual data that will be cascade
	Data interface{}


type ChildRef struct {
	Id bson.ObjectId `bson:"_id" json:"_id"`
	Name string
func (c *Child) GetCascade(collection *bongo.Collection) []*bongo.CascadeConfig {
	connection := collection.Connection
	rel := &ChildRef {
	cascadeSingle := &bongo.CascadeConfig{
		Collection:  connection.Collection("parents").Collection(),
		Properties:  []string{"name"},
		ThroughProp: "child",
		RelType:     bongo.REL_ONE,
		Query: bson.M{
			"_id": c.ParentId,

	cascadeMulti := &bongo.CascadeConfig{
		Collection:  connection.Collection("parents").Collection(),
		Properties:  []string{"name"},
		ThroughProp: "children",
		RelType:     bongo.REL_MANY,
		Query: bson.M{
			"_id": c.ParentId,

	if c.DiffTracker.Modified("ParentId") {

		origId, _ := c.DiffTracker.GetOriginalValue("ParentId")
		if origId != nil {
			oldQuery := bson.M{
				"_id": origId,
			cascadeSingle.OldQuery = oldQuery
			cascadeMulti.OldQuery = oldQuery


	return []*bongo.CascadeConfig{cascadeSingle, cascadeMulti}

This does the following:

  1. When you save a child, it will populate its parent's (defined by cascadeSingle.Query) child property with an object, consisting of one key/value pair (name)

  2. When you save a child, it will also modify its parent's (defined by cascadeMulti.Query) children array, either modifying or pushing to the array of key/value pairs, also with just name.

  3. When you delete a child, it will use cascadeSingle.OldQuery to remove the reference from its previous parent.child

  4. When you delete a child, it will also use cascadeMulti.OldQuery to remove the reference from its previous parent.children

Note that the ThroughProp must be the actual field name in the database (bson tag), not the property name on the struct. If there is no ThroughProp, the data will be cascaded directly onto the root of the document.


Go ODM for MongoDB







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