# Slots

Slots operate the same way as "fields" in traditional object languages and the same ways as "columns" in spreadsheets and relational databases.

If you have a JSON object that is conformant to a LinkML schema, then
the keys for that object must correspond to slots in the schema, that
are applicable to that class.

for example, if we have an object instantiating a Person class:

```json
{"id": "PERSON001",
 "name": "....",
 "email": "....",
 ...
}
```

then `id`, `email`, `name` should all be valid slots, as in the following schema:

```yaml
classes:
  Person:
    slots:
      - id
      - name
      - email
```


If we have tabular data

|id|name|email|
|---|---|---|
|PERSON0001|...|...|

then the same constraints hold.

## ranges

Each slot must have a [range](https://w3id.org/linkml/range) - if this is not declared explicitly, then [default_range](https://w3id.org/linkml/default_range) is used.

The range must be one of:

 * A [ClassDefinition](https://w3id.org/linkml/ClassDefinition), when the value of the slot is a complex object
 * A [TypeDefinition](https://w3id.org/linkml/TypeDefinition), when the value of the slot is an atomic object
 * An [EnumDefinition](https://w3id.org/linkml/EnumDefinition), when the value of the slot is a token that represents a vocabulary element

Examples:

```yaml
slots:
  gender:
    slot_uri: schema:gender
    range: GenderType  ## range is an enum
  has_medical_history:
    range: MedicalEvent ## range is a class
    multivalued: true
    inlined_as_list: true
  age_in_years:
    range: integer  ## range is a type
    minimum_value: 0
    maximum_value: 999
```

## slot_usage

The [slot_usage](https://w3id.org/linkml/slot_usage) slot can be used to refine the meaning of a slot in the context of a particular class.

For example, imagine a schema with a generic "Relationship" class:

```yaml
  Relationship:
    slots:
      - started_at_time
      - ended_at_time
      - related_to
      - type
```

with subtypes such as `FamilialRelationship`, `BusinessRelationship`, etc

we can use `slot_usage` to constrain the meaning of more generic slots such as `type` and `related to`:

```yaml
  FamilialRelationship:
    is_a: Relationship
    slot_usage:
      type:
        range: FamilialRelationshipType
        required: true
      related to:
        range: Person
        required: true
```        

## Identifiers


If a slot is declared as an [identifier](https://w3id.org/linkml/)
then it serves as a unique key for members of that class. It can also
be used for [inlining](inlining) as a dict in JSON serializations.


```yaml
slots:
  id:
    identifier: true
```

the range of an identifier can be any type, but it is a good idea to have these be of type [uriorcurie](https://w3id.org/linkml/uriorcurie)

## Slot cardinality

### multivalued

The [multivalued](https://w3id.org/linkml/multivalued) indicates that the range of the slot is a list

Example:

```yaml
slots:
  has_medical_history:
    range: MedicalEvent
    multivalued: true
    inlined_as_list: true
```


### required

The [required](https://w3id.org/linkml/required) slot can be used to define whether a slot is required.

When a slot is declared as required, any class that uses that slot must have a value for that slot.

### recommended

The [recommended](https://w3id.org/linkml/recommended) slot can be used to define whether a slot is recommended.

If data is missing a recommended slot, it is still considered valid. However, validators may choose to issue warnings.

## inverse

The `inverse` slot can be used to specify the inverse predicate of a given predicate slot relationship.

```yaml
  parent_of:
    is_a: famlially_related_to
    inverse: child_of
```

## logical characteristics

Additional logical characteristics can be specified for a slot; these are all boolean:

* [transitive](https://w3id.org/linkml/transitive)
* [symmetric](https://w3id.org/linkml/symmetric)
* [antisymmetric](https://w3id.org/linkml/symmetric)
* [reflexive](https://w3id.org/linkml/reflexive)
* [locally_reflexive](https://w3id.org/linkml/locally_reflexive)