Empty or not required struct fields in golang

tl;dr: Use ,omitempty, and if you need to worry about the difference between a zero value and null/not specified, do what the GitHub API does and use a pointer.

---

Both json and bson support the ,omitempty tag. For json, "empty values are false, 0, any nil pointer or interface value, and any array, slice, map, or string of length zero" (json docs). For bson, ,omitempty means "Only include the field if it's not set to the zero value for the type or to empty slices or maps", and zero values include empty strings and nil pointers (bson docs).

So if you really need a Group struct, you can put a *Group in instead, and it won't be stored when the pointer is nil. If Investment only needs to hold the group's name, it's even simpler: "" as group name keeps a group key from being stored.

bson defaults to using the lowercased field name already so you can omit that from the struct tag when they match. json will default to the Capitalized name, so specify the lowercase name in a tag if you need lowercase.

So, best case, maybe you can just use:

type Investment struct {
    Base
    Symbol string `json:"symbol" binding:"required"`
    Group string  `json:"group,omitempty" bson:",omitempty"`
    Fields bson.M `json:"fields"`
}

If you ever run into fields where the zero value for the type ("", 0, false, etc.) is distinct from "not specified", you can do what the GitHub API does and put pointers in your structures--essentially an extension of the *Group trick.