There’s currently a bug when using a dedicated class as a policy in
services: if that class delegates its `#call` method (to an underlying
strategy object for example), then an error will be raised saying steps
aren’t allowed to provide default parameters.
This should not happen, and this patch fixes that issue.
Currently, there are two ways (kind of) for accessing `params` inside a
service:
- when there is no contract or it hasn’t been reached yet, `params` is
just the hash that was provided to the service. To access a key, you
have to use the bracket notation `params[:my_key]`.
- when there is a contract and it has been executed successfully,
`params` now references the contract and the attributes are accessible
using methods (`params.my_key`).
This patch unifies how `params` exposes its attributes. Now, even if
there is no contract at all in a service, `params` will expose its
attributes through methods, that way things are more consistent.
This patch also makes sure there is always a `params` object available
even when no `params` key is provided to the service (this allows a
contract to fail because its attributes are blank instead of having the
service raising an error because it doesn’t find `params` in its context).
This patch aims to improve the steps inspector output:
- The service class name is displayed at the top.
- Next to each step is displayed the time it took to run said step.
- Steps that didn’t run are hidden.
- `#inspect` automatically outputs the error when it is present.
This patch adds a new step to services named `try`.
It’s useful to rescue exceptions that some steps could raise. That way,
if an exception is caught, the service will stop its execution and can
be inspected like with any other steps.
Just wrap the steps that can raise with a `try` block:
```ruby
try do
step :step_that_can_raise
step :another_step_that_can_raise
end
```
By default, `try` will catch any exception inheriting from
`StandardError`, but we can specify what exceptions to catch:
```ruby
try(ArgumentError, RuntimeError) do
step :will_raise
end
```
An outcome matcher has been added: `on_exceptions`. By default it will
be executed for any exception caught by the `try` step.
Here also, we can specify what exceptions to catch:
```ruby
on_exceptions(ArgumentError, RuntimeError) do |exception|
…
end
```
Finally, an RSpec matcher has been added:
```ruby
it { is_expected.to fail_with_exception }
# or
it { is_expected.to fail_with_exception(ArgumentError) }
```
We decided to make contracts immutable once their validations have run.
Indeed, it doesn’t make a lot of sense to modify a contract value
outside the contract itself.
If processing is needed, then it should happen inside the contract
itself.
This patch replaces the parameters provided to a service through
`params` by the contract object.
That way, it allows better consistency when accessing input params. For
example, if you have a service without a contract, to access a
parameter, you need to use `params[:my_parameter]`. But with a contract,
you do this through `contract.my_parameter`. Now, with this patch,
you’ll be able to access it through `params.my_parameter` or
`params[:my_parameter]`.
Some methods have been added to the contract object to better mimic a
Hash. That way, when accessing/using `params`, you don’t have to think
too much about it:
- `params.my_key` is also accessible through `params[:my_key]`.
- `params.my_key = value` can also be done through `params[:my_key] =
value`.
- `#slice` and `#merge` are available.
- `#to_hash` has been implemented, so the contract object will be
automatically cast as a hash by Ruby depending on the context. For
example, with an AR model, you can do this: `user.update(**params)`.
Currently in services, we don’t make a distinction between input
parameters, options and dependencies.
This can lead to user input modifying the service behavior, whereas it
was not the developer intention.
This patch addresses the issue by changing how data is provided to
services:
- `params` is now used to hold all data coming from outside (typically
user input from a controller) and a contract will take its values from
`params`.
- `options` is a new key to provide options to a service. This typically
allows changing a service behavior at runtime. It is, of course,
totally optional.
- `dependencies` is actually anything else provided to the service (like
`guardian`) and available directly from the context object.
The `service_params` helper in controllers has been updated to reflect
those changes, so most of the existing services didn’t need specific
changes.
The options block has the same DSL as contracts, as it’s also based on
`ActiveModel`. There aren’t any validations, though. Here’s an example:
```ruby
options do
attribute :allow_changing_hidden, :boolean, default: false
end
```
And here’s an example of how to call a service with the new keys:
```ruby
MyService.call(params: { key1: value1, … }, options: { my_option: true }, guardian:, …)
```
Currently, when calling a service with its block form, a `#result`
method is automatically created on the caller object. Even if it never
clashed so far, this could happen.
This patch removes that method, and instead use a more classical way of
doing things: the result object is now provided as an argument to the
main block. This means if we need to access the result object in an
outcome block, it will be done like this from now on:
```ruby
MyService.call(params) do |result|
on_success do
# do something with the result object
do_something(result)
end
end
```
In the same vein, this patch introduces the ability to match keys from
the result object in the outcome blocks, like we already do with step
definitions in a service. For example:
```ruby
on_success do |model:, contract:|
do_something(model, contract)
end
```
Instead of
```ruby
on_success do
do_something(result.model, result.contract)
end
```
While using `OpenStruct` is nice, it’s generally not a very good idea as
it usually leads to performance problems.
The `OpenStruct` source code even says basically to avoid it.
Since the context object is crucial in our services, this patch replaces
`OpenStruct` with a custom implementation instead.
Currently in services, the `contract` step is only used to define where
the contract will be called in the execution flow. Then, a `Contract`
class has to be defined with validations in it.
This patch allows the `contract` step to take a block containing
validations, attributes, etc. directly. No need to then open a
`Contract` class later in the service.
It also has a nice side effect, as it’s now easy to define multiples
contracts inside the same service. Before, we had the `class_name:`
option, but it wasn’t really useful as you had to redefine a complete
new contract class.
Now, when using a name for the contract other than `default`, a new
contract will be created automatically using the provided name.
Example:
```ruby
contract(:user) do
attribute :user_id, :integer
validates :user_id, presence: true
end
```
This will create a `UserContract` class and use it, also putting the
resulting contract in `context[:user_contract]`.
With the current implementation, a service step can be written as:
```ruby
def my_step(a_default_value: 2)
…
end
```
That’s a pattern we want to avoid as default values (if needed) should
be probably defined in a contract.
This patch makes a service raise an exception if a default value is
encountered.
This will help to enforce a consistent pattern for creating service
actions.
This patch also namespaces actions and policies, making everything
related to a service available directly in
`app/services/<concept-name>`, making things more consistent at that
level too.
Loïc Guitaut