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Filtering, ordering and pagination for Ecto

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# Flop

![CI](https://github.com/woylie/flop/workflows/CI/badge.svg) [![Hex](https://img.shields.io/hexpm/v/flop)](https://hex.pm/packages/flop) [![codecov](https://codecov.io/gh/woylie/flop/branch/main/graph/badge.svg?token=32BSY8O2LI)](https://codecov.io/gh/woylie/flop)

Flop is an Elixir library designed to easily apply filtering, ordering, and
pagination to your Ecto queries.

## Features

- **Offset-based pagination:** Allows pagination through `offset`/`limit` or
  `page`/`page_size` parameters.
- **Cursor-based pagination:** Also known as key set pagination, provides a more
  efficient alternative to offset-based pagination. Compatible with Relay
  pagination arguments.
- **Sorting:** Applies sort parameters on multiple fields in any direction.
- **Filtering:** Allows complex data filtering using multiple conditions,
  operators, and fields.
- **Parameter validation:** Ensures the validity of provided parameters.
- **Configurable filterable and sortable fields:** Only applies parameters to
  the fields that were explicitly configured as filterable or sortable.
- **Join fields:** Allows the application of pagination, sort, and filter
  parameters on any named binding. Provides functions to help you to avoid
  unnecessary join clauses.
- **Compound fields:** Provides the ability to apply filter parameters on
  multiple string fields, for example for a full name filter.
- **Custom fields:** Provides an escape hatch for filtering and ordering that
  Flop is not able to build on its own.
- **Relay connection formatter:** Formats the connection in Relay style,
  providing edges, nodes, and page info.
- **UI helpers and URL builders through
  [Flop Phoenix](https://hex.pm/packages/flop_phoenix):** Pagination, sortable
  tables and filter forms.

## Installation

To get started, add `flop` to your dependencies list in your project's `mix.exs`
file:

```elixir
def deps do
  [
    {:flop, "~> 0.27.2"}
  ]
end
```

Enable debug checks in `config/dev.exs` and `config/test.exs`:

```elixir
config :flop, debug: true
```

You can also configure a default repo for Flop by adding the following line to
`config/config.exs`:

```elixir
config :flop, repo: MyApp.Repo
```

Instead of configuring Flop globally, you can also use a configuration module.
Please refer to the Flop module documentation for more information.

### Compatibility

This package is tested against the Elixir and OTP versions that are still
supported upstream. Older versions down to the requirement in `mix.exs` may
still work, but they are not covered by CI and not officially supported.

### Supported databases

| Database | Status                     |
| -------- | -------------------------- |
| Postgres | Supported and tested in CI |
| MySQL    | Supported and tested in CI |
| SQLite   | Supported and tested in CI |

Flop builds queries with `Ecto.Query` and only uses a couple of fragments to
bridge the gap between databases. The library might work with more Ecto adapter,
but it is only tested with the databases listed above.

Some behavior depends on the used adapter.

#### MySQL

Configure the repo with the charset and the collation your columns use, or
cursor pagination over a string column raises `Illegal mix of collations`:

```elixir
config :my_app, MyApp.Repo,
  charset: "utf8mb4",
  collation: "utf8mb4_0900_ai_ci"
```

MySQL cannot use an index for the array operators, which compile to
`JSON_CONTAINS`, or for the `:asc_nulls_last` and `:desc_nulls_first` order
directions, which add an `IS NULL` sort key because MySQL has neither
`NULLS FIRST` nor `NULLS LAST`.

`{:array, :binary_id}` does not work, because the adapter writes a
`binary_id` as raw bytes and the JSON encoder rejects them. See `Flop.Filter`
for the array operators.

## Usage

### Define sortable and filterable fields

To define sortable and filterable fields in your Ecto schema, you can derive
`Flop.Schema`. This step is optional but highly recommended, particularly when
the parameters passed to Flop's functions are user-provided. Deriving
`Flop.Schema` ensures that Flop applies filtering and sorting parameters only to
the fields you've explicitly configured.

```elixir
defmodule MyApp.Pet do
  use Ecto.Schema

@derive {
    Flop.Schema,
    filterable: [:name, :species],
    sortable: [:name, :age, :species]
  }

schema "pets" do
    field :name, :string
    field :age, :integer
    field :species, :string
    field :social_security_number, :string
  end
end
```

Besides sortable and filterable fields, `Flop.Schema` also allows the definition
of join fields, compound fields, or custom fields. You can also set maximum or
default limits, among other options. For a comprehensive list of available
options, check the `Flop.Schema` documentation.

### Query data

Use the `Flop.validate_and_run/3` or `Flop.validate_and_run!/3` function to both
validate the parameters and fetch data from the database, and acquire pagination
metadata in one operation.

Here is an example of how you might use this in your code:

```elixir
defmodule MyApp.Pets do
  import Ecto.Query, warn: false

alias Ecto.Changeset
  alias MyApp.{Pet, Repo}

@spec list_pets(map) ::
          {:ok, {[Pet.t()], Flop.Meta.t()}} | {:error, Flop.Meta.t()}
  def list_pets(params \\ %{}) do
    Flop.validate_and_run(Pet, params, for: Pet)
  end
end
```

The `for` option sets the Ecto schema for which you derived `Flop.Schema`. If
you haven't derived `Flop.Schema` as described above, this option can be
omitted. However, this is not recommended unless all parameters are generated
internally and are guaranteed to be safe.

On success, `Flop.validate_and_run/3` returns an `:ok` tuple. The second element
of this tuple is another tuple containing the fetched data and metadata.

```elixir
{:ok, {[%Pet{}], %Flop.Meta{}}}
```

You can learn more about the `Flop.Meta` struct in the
[module documentation](https://hexdocs.pm/flop/Flop.Meta.html).

Alternatively, you may separate parameter validation and data fetching into
different steps using the Flop.validate/2, Flop.validate!/2, and Flop.run/3
functions. This allows you to manipulate the validated parameters, to modify the
query depending on the parameters, or to move the parameter validation to a
different layer of your application.

```elixir
with {:ok, flop} <- Flop.validate(params, for: Pet) do
  Flop.run(Pet, flop, for: Pet)
end
```

The aforementioned functions internally call the lower-level functions
`Flop.all/3`, `Flop.meta/3`, and `Flop.count/3`. If you have advanced
requirements, you might prefer to use these functions directly. However, it's
important to note that these lower-level functions do not validate the
parameters. If parameters are generated based on user input, they should always
be validated first using `Flop.validate/2` or `Flop.validate!/2` to ensure safe
execution.

The examples above assume that you configured a default repo. However, you can
also pass the repo directly to the functions:

```elixir
Flop.validate_and_run(Pet, flop, repo: MyApp.Repo)
Flop.all(Pet, flop, repo: MyApp.Repo)
Flop.meta(Pet, flop, repo: MyApp.Repo)
```

For more detailed information, refer the
[documentation](https://hexdocs.pm/flop/readme.html).

## Parameter format

The Flop library requires parameters to be provided in a specific format as a
map. This map can be translated into a URL query parameter string, typically
for use in a web framework like Phoenix.

### Pagination

#### Offset / limit

You can specify an offset to start from and a limit to the number of results.

```elixir
%{offset: 20, limit: 10}
```

This translates to the following query parameter string:

```html
?offset=20&limit=10
```

#### Page / page size

You can specify the page number and the size of each page.

```elixir
%{page: 2, page_size: 10}
```

This translates to the following query parameter string:

```html
?page=2&page_size=10
```

#### Cursor

You can fetch a specific number of results before or after a given cursor.

```elixir
%{first: 10, after: "g3QAAAABZAACaWRiAAACDg=="}
%{last: 10, before: "g3QAAAABZAACaWRiAAACDg=="}
```

These translate to the following query parameter strings:

```html
?first=10&after=g3QAAAABZAACaWRiAAACDg==
?last=10&before=g3QAAAABZAACaWRiAAACDg==
```

The cursor pagination arguments are based on the
[GraphQL Cursor Connection Specification, section 4](https://relay.dev/graphql/connections.htm#sec-Arguments).

### Ordering

To sort the results, specify fields to order by and the direction of sorting for
each field.

```elixir
%{order_by: [:name, :age], order_directions: [:asc, :desc]}
```

This translates to the following query parameter string:

```html
?order_by[]=name&order_by[]=age&order_directions[]=asc&order_directions[]=desc
```

### Filters

You can filter the results by providing a field, an operator, and a value. The
operator is optional and defaults to `==`. Multiple filters are combined with a
logical `AND`. At the moment, combining filters with `OR` is not supported.

```elixir
%{filters: [%{field: :name, op: :ilike_and, value: "Jane"}]}
```

This translates to the following query parameter string:

```html
?filters[0][field]=name&filters[0][op]=ilike_and&filters[0][value]=Jane
```

Refer to the `Flop.Filter` documentation and `t:Flop.t/0` type documentation for
more details on using filters.

## Internal parameters

Flop is designed to manage parameters that come from the user side. While it is
possible to alter those parameters and append extra filters upon receiving them,
it is advisable to clearly differentiate parameters coming from outside and the
parameters that your application adds internally.

Consider the scenario where you need to scope a query based on the current user.
In this case, it is better to create a separate function that introduces the
necessary `WHERE` clauses:

```elixir
def list_pets(%{} = params, %User{} = current_user) do
  Pet
  |> scope(current_user)
  |> Flop.validate_and_run(params, for: Pet)
end

defp scope(q, %User{role: :admin}), do: q
defp scope(q, %User{id: user_id}), do: where(q, user_id: ^user_id)
```

If you need to add extra filters that are only used internally and aren't
exposed to the user, you can pass them as a separate argument. This same
argument can be used to override certain options depending on the context in
which the function is called.

```elixir
def list_pets(%{} = params, opts \\ [], %User{} = current_user) do
  flop_opts =
    opts
    |> Keyword.take([
      :default_limit,
      :default_pagination_type,
      :pagination_types
    ])
    |> Keyword.put(:for, Pet)

Pet
  |> scope(current_user)
  |> apply_filters(opts)
  |> Flop.validate_and_run(params, flop_opts)
end

defp scope(q, %User{role: :admin}), do: q
defp scope(q, %User{id: user_id}), do: where(q, user_id: ^user_id)

defp apply_filters(q, opts) do
  Enum.reduce(opts, q, fn
    {:last_health_check, dt}, q -> where(q, [p], p.last_health_check < ^dt)
    {:reminder_service, bool}, q -> where(q, [p], p.reminder_service == ^bool)
    _, q -> q
  end)
end
```

With this approach, you maintain a clean separation between user-driven
parameters and system-driven parameters, leading to more maintainable and less
error-prone code.

## Relay and Absinthe

The `Flop.Relay` module is useful if you are using
[absinthe](https://hex.pm/packages/absinthe) with
[absinthe_relay](https://hex.pm/packages/absinthe_relay), or if you simply need
to adhere to the Relay cursor specification. This module provides functions that
help transform query responses into a format compatible with Relay.

Consider the scenario where you have defined node objects for owners and pets,
along with a connection field for pets on the owner node object.

```elixir
node object(:owner) do
  field :name, non_null(:string)
  field :email, non_null(:string)

connection field :pets, node_type: :pet do
    resolve &MyAppWeb.Resolvers.Pet.list_pets/2
  end
end

node object(:pet) do
  field :name, non_null(:string)
  field :age, non_null(:integer)
  field :species, non_null(:string)
end

connection(node_type: :pet)
```

Absinthe Relay will establish the arguments `after`, `before`, `first` and
`last` on the `pets` field. These argument names align with those used by Flop, facilitating their application.

Next, we'll define a `list_pets_by_owner/2` function in the Pets context.

```elixir
defmodule MyApp.Pets do
  import Ecto.Query

alias MyApp.{Owner, Pet, Repo}

@spec list_pets_by_owner(Owner.t(), map) ::
          {:ok, {[Pet.t()], Flop.Meta.t()}} | {:error, Flop.Meta.t()}
  def list_pets_by_owner(%Owner{id: owner_id}, params \\ %{}) do
    Pet
    |> where(owner_id: ^owner_id)
    |> Flop.validate_and_run(params, for: Pet)
  end
end
```

Now, within your resolver, you merely need to invoke the function and call
`Flop.Relay.connection_from_result/1`, which transforms the result into a tuple
composed of the edges and the page_info, as required by `absinthe_relay`.

```elixir
defmodule MyAppWeb.Resolvers.Pet do
  alias MyApp.{Owner, Pet}

def list_pets(args, %{source: %Owner{} = owner} = resolution) do
    with {:ok, result} <- Pets.list_pets_by_owner(owner, args) do
      {:ok, Flop.Relay.connection_from_result(result)}
    end
  end
end
```

In case you want to introduce additional filter arguments, you can employ
`Flop.nest_filters/3` to convert simple filter arguments into Flop filters,
without necessitating API users to understand the Flop filter format.

Let's add `name` and `species` filter arguments to the `pets` connection field.

```elixir
node object(:owner) do
  field :name, non_null(:string)
  field :email, non_null(:string)

connection field :pets, node_type: :pet do
    arg :name, :string
    arg :species, :string

resolve &MyAppWeb.Resolvers.Pet.list_pets/2
  end
end
```

Assuming that these fields have been already configured as filterable with
`Flop.Schema`, we can use `Flop.nest_filters/3` to take the filter arguments and
transform them into a list of Flop filters.

```elixir
defmodule MyAppWeb.Resolvers.Pet do
  alias MyApp.{Owner, Pet}

def list_pets(args, %{source: %Owner{} = owner} = resolution) do
    args = nest_filters(args, [:name, :species])

with {:ok, result} <- Pets.list_pets_by_owner(owner, args) do
      {:ok, Flop.Relay.connection_from_result(result)}
    end
  end
end
```

`Flop.nest_filters/3` uses the equality operator `:==` by default.
You can override the default operator per field.

```elixir
args = nest_filters(args, [:name, :species], operators: %{name: :ilike_and})
```

## Flop Phoenix

[Flop Phoenix](https://hex.pm/packages/flop_phoenix) is a companion library that
provides Phoenix components for pagination, sortable tables, and filter forms,
usable with both Phoenix LiveView and in dead views. It also defines helper
functions to build URLs with Flop query parameters.

## Development

Pull requests are welcome, but for non-trivial changes like new features or
API changes, please open an issue first and describe what you plan to do.
Otherwise, you risk spending time on development work that might not be
accepted.

The database tests require running Postgres and MySQL instances. SQLite runs
from a file and needs nothing. There is a docker compose configuration you can
use:

```bash
docker compose up
```

Please ensure that the following checks pass before requesting a review:

```bash
mix format --check-formatted
mix credo
mix compile --warnings-as-errors
mix test --warnings-as-errors
mix hex.audit
mix docs --warnings-as-errors
mix dialyzer
```

Note that the tests are split up. The common functionality is tested without
database interaction, and each Ecto adapter has its own suite that runs the
tests involving database queries.

`mix test` runs the common tests and the adapters that are expected to pass,
which is every adapter at the moment.

- `mix test` - common functionality and all supported adapters
- `mix test.base` - common functionality only (runs without database)
- `mix test.postgres`
- `mix test.mysql`
- `mix test.sqlite`

`mix test` is required to pass.

For general contribution guidelines, refer to
https://github.com/woylie/.github/blob/main/CONTRIBUTING.md.

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