# Calculations
Calculations in Ash allow for displaying complex values as a top level value of a resource.
## Primer
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## Declaring calculations on a resource
### Expression Calculations
The simplest kind of calculation refers to an Ash expression. For example:
```elixir
calculations do
calculate :full_name, :string, expr(first_name <> " " <> last_name)
end
```
See the [Expressions guide](/documentation/topics/expressions.md) for more.
### Module Calculations
When calculations require more complex code or can't be pushed down into the data layer, a module that uses `Ash.Calculation` can be used.
```elixir
defmodule Concat do
# An example concatenation calculation, that accepts the delimiter as an argument,
#and the fields to concatenate as options
use Ash.Calculation
# Optional callback that verifies the passed in options (and optionally transforms them)
@impl true
def init(opts) do
if opts[:keys] && is_list(opts[:keys]) && Enum.all?(opts[:keys], &is_atom/1) do
{:ok, opts}
else
{:error, "Expected a `keys` option for which keys to concat"}
end
end
@impl true
# A callback to tell Ash what keys must be loaded/selected when running this calculation
def load(_query, opts, _context) do
opts[:keys]
end
@impl true
def calculate(records, opts, %{separator: separator}) do
Enum.map(records, fn record ->
Enum.map_join(opts[:keys], separator, fn key ->
to_string(Map.get(record, key))
end)
end)
end
# You can implement this callback to make this calculation possible in the data layer
# *and* in elixir. Ash expressions are already executable in Elixir or in the data layer, but this gives you fine grain control over how it is done
# @impl true
# def expression(opts, context) do
# end
end
# Usage in a resource
calculations do
calculate :full_name, :string, {Concat, keys: [:first_name, :last_name]} do
# You need to use the [allow_empty?: true, trim?: false] constraints here.
# The separator could be an empty string or require a leading or trailing space,
# but would be trimmed or even set to `nil` without the constraints shown below.
argument :separator, :string do
allow_nil? false
constraints [allow_empty?: true, trim?: false]
default ""
end
end
end
```
See the documentation for the calculations section in [Resource DSL docs](dsl-ash-resource.html#calculations) and the `Ash.Calculation` docs for more information.
The calculations declared on a resource allow for declaring a set of named calculations that can be used by extensions.
They can also be loaded in the query using `Ash.Query.load/2`, or after the fact using `c:Ash.Api.load/3`. Calculations declared on the resource will be keys in the resource's struct.
## Custom calculations in the query
Example:
```elixir
User
|> Ash.Query.new()
|> Ash.Query.calculate(:full_name, {Concat, keys: [:first_name, :last_name]}, :string, %{separator: ","})
```
See the documentation for `Ash.Query.calculate/4` for more information.
## Arguments in calculations
Using the above example with arguments, you can load a calculation with arguments like so:
```elixir
load(full_name: [separator: ","])
```
If the calculation uses an expression, you can also filter and sort on it like so:
```elixir
query
|> Ash.Query.filter(full_name(separator: ","))
|> Ash.Query.sort(full_name: {:asc, %{separator: ","}})
```
