Eager creation failure for associations

[xixinjie]

Description

When try to use db.Eager().ValidateAndCreate(user) to create a user and underlying has_many books, it succeeded for the first time when books table is empty, but failed to save the books in following creations.

Steps to Reproduce the Problem

1. Create new users and books tables
2. Use db.Eager().ValidateAndCreate(user) to create first user and underlying has_many books: success
3. Use db.Eager().ValidateAndCreate(user) to create more users and underlying has_many books: failure

Expected Behavior

Should always be able create the users and associated books via db.Eager().ValidateAndCreate(user)

Actual Behavior

It succeeded for the very first creation, but failed for following creations.

Info

OS: Mac
Pop: v5.3.1
Buffalo: N/A

More details could be found in the PR #607

@markbates @stanislas-m Could you pls review the PR?