Composite keys

You need composite keys when you have multiple values that identify a particular object. For example, say you are dealing with a system where users may have multiple sessions. You can store this information in a map of maps, or you can create a composite key containing the user ID and session ID.

How to do it...

Use a comparable struct or an array as the map key. A comparable struct is, in general, a struct that does not contain the following:

• Slices
• Channels
• Functions
• Maps
• Other non-comparable structs

So, to use composite keys, perform the following steps:

1. Define a comparable struct:

type Key struct {
  UserID string
  SessionID string
}
type User struct {
  Name string
  ...
}
var compositeKeyMap = map[Key]User{}

2. Use an instance of the map key to access elements:

compositeKeyMap[Key{
  UserID: "123",
  SessionID: "1",
 ...