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78 lines
2.6 KiB
Markdown
78 lines
2.6 KiB
Markdown
# Attributes
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Attributes specify the `name`, `type` and additional configuration of a simple property of a record. When using SQL data layers, for example, an attribute would correspond to a column in a database table. For information on types, see `Ash.Type`.
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To see all of the options available when building attributes, see `d:Ash.Resource.Dsl.attributes.attribute`
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If you are looking to compute values on demand, see the [Calculations guide](/documentation/topics/resources/calculations.md) and the [aggregates guide](/documentation/topics/resources/aggregates.md).
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## Special attributes
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In Ash there are 4 special attributes these are:
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- `create_timestamp`
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- `update_timestamp`
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- `integer_primary_key`
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- `uuid_primary_key`
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These are really just shorthand for an attribute with specific options set. They're outlined below.
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### `create_timestamp`
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You may recognise this if you have used Ecto before. This attribute will record the time at which each row is created, by default it uses `DateTime.utc_now/1`.
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`create_timestamp :inserted_at` is equivalent to an attribute with these options:
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```elixir
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attribute :inserted_at, :utc_datetime_usec do
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writable? false
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default &DateTime.utc_now/0
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match_other_defaults? true
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allow_nil? false
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end
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```
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### `update_timestamp`
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This is also similar in Ecto. This attribute records the last time a row was updated, also using `DateTime.utc_now/1` by default.
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`update_timestamp :updated_at` is equivalent to:
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```elixir
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attribute :updated_at, :utc_datetime_usec do
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writable? false
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default &DateTime.utc_now/0
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update_default &DateTime.utc_now/0
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match_other_defaults? true
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allow_nil? false
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end
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```
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### `uuid_primary_key`
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This attribute is used in almost every resource. It generates a UUID every time a new record is made.
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`uuid_primary_key :id` is equivalent to:
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```elixir
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attribute :id, :uuid do
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writable? false
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default &Ash.UUID.generate/0
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primary_key? true
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allow_nil? false
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end
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```
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### `integer_primary_key`
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Creates a generated integer primary key. Keep in mind that not all data layers support auto incrementing ids, but for SQL data layers this is a very common practice. For those that don't, it is your own job to provide values for the primary key. We generally suggest using UUIDs over integers, as there are [a lot of good reasons to not use autoincrementing integer ids](https://www.clever-cloud.com/blog/engineering/2015/05/20/why-auto-increment-is-a-terrible-idea/).
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`integer_primary_key :id` is equivalent to:
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```elixir
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attribute :id, :integer do
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writable? false
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generated? true
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primary_key? true
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allow_nil? false
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end
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```
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