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# API styleguide
This styleguide recommends best practices for API development.
## Instance variables
Please do not use instance variables, there is no need for them (we don't need
to access them as we do in Rails views), local variables are fine.
## Entities
Always use an [Entity] to present the endpoint's payload.
## Methods and parameters description
Every method must be described using the [Grape DSL](https://github.com/ruby-grape/grape#describing-methods)
(see <https://gitlab.com/gitlab-org/gitlab-foss/blob/master/lib/api/environments.rb>
for a good example):
- `desc` for the method summary. You should pass it a block for additional
details such as:
- The GitLab version when the endpoint was added
- If the endpoint is deprecated, and if so, when will it be removed
- `params` for the method params. This acts as description,
[validation, and coercion of the parameters]
A good example is as follows:
```ruby
desc 'Get all broadcast messages' do
detail 'This feature was introduced in GitLab 8.12.'
success Entities::BroadcastMessage
end
params do
optional :page, type: Integer, desc: 'Current page number'
optional :per_page, type: Integer, desc: 'Number of messages per page'
end
get do
messages = BroadcastMessage.all
present paginate(messages), with: Entities::BroadcastMessage
end
```
## Declared params
> Grape allows you to access only the parameters that have been declared by your
`params` block. It filters out the params that have been passed, but are not
allowed.
– <https://github.com/ruby-grape/grape#declared>
### Exclude params from parent namespaces
> By default `declared(params)`includes parameters that were defined in all
parent namespaces.
– <https://github.com/ruby-grape/grape#include-parent-namespaces>
In most cases you will want to exclude params from the parent namespaces:
```ruby
declared(params, include_parent_namespaces: false)
```
### When to use `declared(params)`
You should always use `declared(params)` when you pass the params hash as
arguments to a method call.
For instance:
```ruby
# bad
User.create(params) # imagine the user submitted `admin=1`... :)
# good
User.create(declared(params, include_parent_namespaces: false).to_h)
```
>**Note:**
`declared(params)` return a `Hashie::Mash` object, on which you will have to
call `.to_h`.
But we can use `params[key]` directly when we access single elements.
For instance:
```ruby
# good
Model.create(foo: params[:foo])
```
## Using API path helpers in GitLab Rails codebase
Because we support [installing GitLab under a relative URL], one must take this
into account when using API path helpers generated by Grape. Any such API path
helper usage must be in wrapped into the `expose_path` helper call.
For instance:
```haml
- endpoint = expose_path(api_v4_projects_issues_related_merge_requests_path(id: @project.id, issue_iid: @issue.iid))
```
[Entity]: https://gitlab.com/gitlab-org/gitlab-foss/blob/master/lib/api/entities.rb
[validation, and coercion of the parameters]: https://github.com/ruby-grape/grape#parameter-validation-and-coercion
[installing GitLab under a relative URL]: https://docs.gitlab.com/ee/install/relative_url.html
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