Sadness is our primaryassociation with crying, but the fact is that people report feeling happier after crying.
悲伤是我们哭泣的主要原因，但事实上，人们流泪后会感觉更快乐。
2
That you tie to the primaryassociation.
来绑定你的基本关联。
3
When we are able to relate without ambiguity any association end to a foreign key, we can apply the correspondence to specify to which primary key the foreign key refers.