Organizing a multiple-file Go project with private support files

Following this answer I have created the following project structure:

.
├── bin
├── pkg
└── src
    └── github.com
        └── GITHUB_USERNAME
            └── PROJECTNAME
                ├── lib
                │   └── model.go
                │   └── ... .go
                ├── LICENSE
                ├── README.md
                └── PROJECTNAME.go

• PROJECTNAME.go has the package main
• model.go has the package PROJECTNAME

In the PROJECTNAME.go I am importing the follwing:

import(
    'github.com/GITHUB_USERNAME/PROJECTNAME/lib/model'
) 

but when I run go build I get the follwing error:

 cannot find package "github.com/GITHUB_USERNAME/PROJECTNAME/lib/model" in any of:
	/usr/lib/go/src/pkg/github.com/GITHUB_USERNAME/PROJECTNAME/lib/model (from $GOROOT)
	/home/USERNAME/go/src/github.com/GITHUB_USERNAME/PROJECTNAME/lib/model (from $GOPATH)

How must the packages names be to import correctly? Are there any other strategies?

Two things here:

1. You missed "project" during the import. Use github.com/GITHUB_USERNAME/PROJECTNAME/lib/model
2. Minor: Naming the package github.com/GITHUB_USERNAME/PROJECTNAME/lib/model "PROJECTNAME" seems strange. How about lib or model?

Your import statement from your main package should read

import(
    "github.com/GITHUB_USERNAME/PROJECTNAME/lib/PROJECTNAME"
) 

You said your model.go uses PROJECTNAME as the package. So you don't actually name a file in your import path. The last component should be the package name. That means you can have a number of go files in the lib directory all with PROJECTNAME as the package.

Package import paths are defined by directory names, not file names.

The correct import path is "github.com/GITHUB_USERNAME/PROJECTNAME/lib" and all of the go files in that folder must have the same package clause at the top.

The identifier after the package clause is what identifier will be imported into the package that imports it. So if it's package foo, you can access the Bar identifier by foo.Bar in the importing code.

By convention, authors typically use the last part of the import path as the package name, so in this case, you should have package lib at the top of all of the Go files under the lib folder.