Model has-one-through relations

[zsilbi]

Hello!

• Type: new feature

In raising this pull request, I confirm the following:

• I have read and understood the Contributing Guidelines
• I have checked that another pull request for this purpose does not exist
• I wrote some tests for this PR
• I have updated the relevant CHANGELOG
• I have created a PR for the documentation about this change

Small description of change:

This is very similar to hasManyToMany, but it can only use one intermediate row.
The related record is either ModelInterface, or null.

This is kind of an antipattern, but it can be useful when you have to work with a poorly designed database structure that can't be altered.

Thanks,
zsilbi

[codecov]

Codecov Report

Merging #14511 into 4.0.x will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            4.0.x   #14511      +/-   ##
==========================================
- Coverage   67.74%    67.7%   -0.05%     
==========================================
  Files         482      482              
  Lines      111424   111754     +330     
==========================================
+ Hits        75483    75659     +176     
- Misses      35941    36095     +154

[niden]

Thank you @zsilbi

[scrnjakovic]

@zsilbi Could you explain why it's anti-pattern? I genuinely want to know. I thought it's perfectly normal to design database that way if you want to have an optional relationship and normalized database, i.e.

document
document_id (PK)
...

document_approved_by
document_id (PK)
user_id (FK)

user
user_id (PK)

instead of

document
document_id (PK)
approved_by (FK) nullable

Thanks and also thanks for PR :)

[zsilbi]

@zsilbi Could you explain why it's anti-pattern?

What I think I really meant there, that performance wise there can be better alternatives to this.
But I also started to use it in some distinct cases, and I also see the beauty in your example. 😄