# Params
Easily define parameter structure and validate/cast with [Ecto.Schema][Ecto.Schema]
[](https://travis-ci.org/vic/params)
- [About](#about)
- [Installation](#installation)
- [Usage](#usage)
- [API Documentation](https://hexdocs.pm/params/)
## Installation
[Available in Hex](https://hex.pm/packages/params), the package can be installed as:
1. Add params to your list of dependencies in `mix.exs`:
```elixir
def deps do
[{:params, "~> 2.0.2"}]
end
```
## About
If you've been doing [Ecto][Ecto] based applications lately,
you know Ecto provides a very easy way to populate structs with data comming
from request parameters, validating and casting their values along the way.
All this thanks to the [Ecto.Schema][Ecto.Schema] and [Ecto.Changeset][cast] modules.
The first specifies the fields your model has (typically the same as your db table)
and the later provides an easy way to convert potentially unsafe data and validate
stuff via changesets.
So for example, in a typical [Phoenix][Phoenix] application, a `User` model
would look like:
```elixir
defmodule MyApp.User do
use MyApp.Web, :model
schema "users" do
field :name, :string
field :age, :integer
end
@required [:name]
@optional [:age]
def changeset(changeset_or_model, params) do
cast(changeset_or_model, params, @required ++ @optional)
|> validate_required(@required)
end
end
```
Normally, changesets are related to some data that will be persisted into
a database, and your controller would use the `User.changeset` method like:
```elixir
# UserController.ex
def create(conn, params) do
ch = User.changeset(%User{}, params)
if ch.valid? do
...
end
```
However, you can use `Ecto.Schema` for validating/casting data that
*wont necessarily* be persisted into a database. All you need is just specify a module and
define your schema, [Ecto.Changeset][cast] will be happy to work with it.
This comes handy when you have certain parameter structure you want
to enforce for example when creating a REST API.
Some Rails developers might be right now wondering where their
_strong parameters_ can be defined. On Elixir land, there's no need for such a thing, as we will see, just using an `Ecto.Schema` with `Ecto.Changeset`
can be much more flexible. Using schemas allows not only
specifing which fields we want, but changesets let use
type cast, perform validations on values, etc.
So, for example, suppose your Phoenix based API performs a search for kittens looking for a
home and expects something like:
```json
{
"breed": "Russian Blue",
"age_min": 0,
"age_max": 5,
"near_location": {
"latitude": 92.1,
"longitude": -82.1
}
}
```
You'd like to validate that your controller has received the correct
params structure, all you need to do is create a couple of modules:
```elixir
defmodule MyApi.Params.Location
use Ecto.Schema
import Ecto.Changeset
@required ~w(latitude longitude)
@optional ~w()
schema "location params" do
field :latitude, :float
field :longitude, :float
end
def changeset(ch, params) do
cast(ch, params, @required ++ @optional)
|> validate_required(@required)
end
end
defmodule MyAPI.Params.KittenSearch
use Ecto.Schema
import Ecto.Changeset
@required ~w(breed)
@optional ~w(age_min age_max)
schema "params for kitten search" do
field :breed, :string
field :age_min, :integer
field :age_max, :integer
embeds_one :near_location, Location
end
def changeset(ch, params) do
cast(ch, params, @required ++ @optional)
|> validate_required(@required)
|> cast_embed(:near_location, required: true)
end
end
# On your controller:
def search(conn, params) do
alias MyAPI.Params.KittenSearch
changeset = KittenSearch.changeset(%KittenSearch{}, params)
if changeset.valid? do
...
end
```
That would allow you to take only valid params as you'd
normally have with any other Ecto.Schema module.
However it's still a lot of code, most of it
defining the the changeset, specifying the optional
and required fields, etc.
[Params](#usage) is just a simple [Ecto.Schema][Ecto.Schema]
wrapper for reducing all this boilerplate, while still
leting you create custom changesets for parameter processing.
## Usage
The previous example could be written like:
```elixir
defmodule MyAPI.KittenController do
use Params
defparams kitten_search %{
breed!: :string,
age_max: :integer,
age_min: [field: :integer, default: 1],
near_location!: %{
latitude!: :float, longitude!: :float
},
tags: [:string]
}
def index(conn, params) do
changeset = kitten_search(params)
if changeset.valid? do
search = Params.data changeset
IO.puts search.near_location.latitude
...
end
end
```
The `defparams` macro generates a module for processing
a [params schema](http://hexdocs.pm/params/Params.Schema.html)
By default all fields are optional. You can mark
required fields by ending them with a `!`, of course
the bang is removed from the field definition and is
only used to mark which fields are required by default.
You can also create a module and define
your schema or custom changesets in it:
```elixir
defmodule UserSearch do
use Params.Schema, %{name: :string, age: :integer}
import Ecto.Changeset, only: [cast: 3, validate_inclusion: 3]
def child(ch, params) do
cast(ch, params, ~w(name age))
|> validate_inclusion(:age, 1..6)
end
end
defmodule MyApp.UserController do
def index(conn, params) do
changeset = UserSearch.from(params, with: &UserSearch.child/2)
if changeset.valid? do
# age in 1..6
end
end
```
The [Params.data](http://hexdocs.pm/params/Params.html#data/1)
and [Params.to_map](http://hexdocs.pm/params/Params.html#to_map/1) can be useful
for obtaining a struct or map from a changeset.
Note that `Params.data` and `Params.to_map` have different behaviour: `data`
returns a struct which will include all valid params. `to_map` returns a map
that only includes the submitted keys and keys with default values:
```elixir
defmodule UserUpdateParams do
use Params.Schema, %{
name: :string,
age: :integer,
auditlog: [field: :boolean, default: true]
}
end
changeset = UserUpdateParams.from(%{name: "John"})
Params.data(changeset) # => %UserUpdateParams{name: "John", age: nil, auditlog: true}
Params.to_map(changeset) # => %{name: "John", auditlog: true}
```
## API Documentation
[API Documentation](https://hexdocs.pm/params/)
[Phoenix]: http://www.phoenixframework.org
[Ecto]: https://hexdocs.pm/ecto
[Ecto.Schema]: https://hexdocs.pm/ecto/Ecto.Schema.html
[cast]: https://hexdocs.pm/ecto/Ecto.Changeset.html#cast/3
## Contributors
- Lasse Skindstad Ebert @lasseebert
- Victor Borja @vborja
