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Revert "Merge pull request #33385 from lanzhiheng/add-example-for-has-and-belongs-to-many-association"

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This reverts commit 3c8c410012, reversing
changes made to daee94da99.

We have this information below in the reference section for
has_and_belongs_to_many.
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Rafael Mendonça França 2018-07-18 16:31:42 -04:00
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guides/source/association_basics.md
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@ -365,22 +365,6 @@ class CreateAssembliesAndParts < ActiveRecord::Migration[5.0]
end
```
As you can see we don't create the relevant model for `assemblies_parts` table, So We can't create the many-to-many record like this
``` ruby
AssemblyPart.create(assembly: @assembly, part: @part) # => NameError: uninitialized constant AssemblyPart
```
If you want to create the relevant many-to-many record, you can use the below code
``` ruby
@assembly.parts << @part
# Or
@part.assemblies << @assembly
```
They are equivalent.
### Choosing Between `belongs_to` and `has_one`
If you want to set up a one-to-one relationship between two models, you'll need to add `belongs_to` to one, and `has_one` to the other. How do you know which is which?