如何使用mgo在单个集合中处理多种类型

Question

I'm very new to golang and try to write a simple event-sourcing user-management webapi using mongodb as backing database. Now i have User, which looks something like this:

type User struct {
Id       bson.ObjectId `json:"id" bson:"_id"`
UserName string        `json:"username" bson:"username"`
Email    string        `json:"email" bson:"email"`
PwdHash  string        `json:"pwd_hash" bson:"pwd_hash"`
FullName string        `json:"fullname" bson:"fullname"`
}

and three events, happening to user, when somebody uses api:

type UserCreatedEvent struct {
    UserId         bson.ObjectId `json:"id" bson:"_id"`
    //time when event was issued
    CreatedEpoch   time.Time     `json:"created_epoch" bson:"created_epoch"`
    //id of user that made a change
    IssuedByUserId bson.ObjectId `json:"issuedby_userid" bson:"issuedby_userid"`
}

type UserDeletedEvent struct {
    UserId         bson.ObjectId `json:"id" bson:"_id"`
    CreatedEpoch   time.Time     `json:"created_epoch" bson:"created_epoch"`
    //id of user that made a change
    IssuedByUserId bson.ObjectId `json:"issuedby_userid" bson:"issuedby_userid"`
}

type UserUpdatedEvent struct {
    UserId         bson.ObjectId `json:"id" bson:"_id"`
    CreatedEpoch   time.Time     `json:"created_epoch" bson:"created_epoch"`
    //id of user that made a change
    IssuedByUserId bson.ObjectId `json:"issuedby_userid" bson:"issuedby_userid"`
    ChangedFieldName     string  `json:"changed_field_name" bson:"changed_field_name"`
    NewChangedFieldValue string  `json:"new_changed_field_value" bson:"new_changed_field_value"`
}

Now i'm stuck on saving and retrieving events from db. The problem is i want to store them in a single collection, so that i have a full plain history of user modifications. But i can't find how to correctly store event type name as a mongo document field and then use it in searches. What is the idiomatic go-way to do this?

I'll be gratefull for any help.

Answer 1

What's nice about a non-relational database is that a little redundancy is OK and is faster for retrieval since you're not joining things together. You could just add your bottom three objects as properties on your User in whatever fashion makes sense for you and your data. Then you wouldn't need to store the UserId on those objects either.

If you need to search over the Events quickly, you could create another collection to hold them. You'd be inserting to two collections, but retrieval times/logic should be pretty good/easy to write.

Your new Event would be something like:

type UserEventType int

const (
    Created UserEventType = iota
    Deleted
    Updated
)

type UserEvent struct {
    UserId               bson.ObjectId `json:"id" bson:"_id"`
    CreatedEpoch         time.Time     `json:"created_epoch" bson:"created_epoch"`
    //id of user that made a change
    IssuedByUserId       bson.ObjectId `json:"issuedby_userid" bson:"issuedby_userid"`
    ChangedFieldName     string        `json:"changed_field_name,omitempty" bson:"changed_field_name,omitempty"`
    NewChangedFieldValue string     `json:"new_changed_field_value,omitempty" bson:"new_changed_field_value,omitempty"`
    EventType            UserEventType `json:"user_event_type" bson:"user_event_type"`
}

Notice the omitempty on the fields that are optional depending on the type of event.

You really aren't supposed to store different objects in the same collection. Here's a line from the Mongo documentation:

MongoDB stores documents in collections. Collections are analogous to tables in relational databases.

In case you aren't familiar with relational databases, a table would essentially represent one type of object.