Associations now raise `ArgumentError` on name conflicts.

Dangerous association names conflicts include instance or class
methods already defined by `ActiveRecord::Base`.
Remove unused variable.
Updated association_basics.md [ci skip]

Updated association_basics.md to include how to write migrations for self joins

Update association_basics.md

Corrected the update

Update association_basics.md

Typo fix

Changed :employee to :manager
Merge pull request #13898 from timfenney/my_branch

Remove unused variable.
Document default trim mode for Erubis and affected ERB tags [ci skip]
Fixes #12963
Merge pull request #13896 from laurocaetano/dangerous_association_names

Associations now raises `ArgumentError` on name conflicts. Closes #13217.
Merge pull request #13900 from zzak/issue_12963

Document default trim mode for Erubis and affected ERB tags. Fixes #12963 [ci skip]
Minor changelog improvements [ci skip]
Merge pull request #13780 from rohitpaulk/patch-1

Updated association_basics.md