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Clarify how primary keys interact with value generation #1907

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Nov 10, 2019
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Clarify how primary keys interact with value generation #1907

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9 changes: 5 additions & 4 deletions entity-framework/core/modeling/generated-properties.md
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@@ -1,8 +1,9 @@
---
title: Generated Values - EF Core
author: rowanmiller
ms.date: 10/27/2016
ms.assetid: eb082011-11a1-41b4-a108-15daafa03e80
description: How to configure value generation for properties when using Entity Framework Core
author: AndriySvyryd
ms.author: ansvyryd
ms.date: 11/06/2019
uid: core/modeling/generated-properties
---

Expand Down Expand Up @@ -48,7 +49,7 @@ Like `value generated on add`, if you specify a value for the property on a newl

## Conventions

By convention, non-composite primary keys of type short, int, long, or Guid will be setup to have values generated on add. All other properties will be setup with no value generation.
By default, non-composite primary keys of type short, int, long, or Guid will be setup to have values generated on add. All other properties will be setup with no value generation.
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Is it still true that you don't get value generation if the property is part of a composite? I vaguely remember that we (possibly unintentionally) changed this.

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Yes, we changed this back for non-owned types. And I don't think we need to talk about owned types here.


## Data Annotations

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23 changes: 18 additions & 5 deletions entity-framework/core/modeling/keys.md
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---
title: Keys (primary) - EF Core
author: rowanmiller
ms.date: 10/27/2016
ms.assetid: 912ffef7-86a0-4cdc-a776-55f907459d20
description: How to configure keys for entity types when using Entity Framework Core
author: AndriySvyryd
ms.author: ansvyryd
ms.date: 11/06/2019
uid: core/modeling/keys
---
# Keys (primary)
Expand All @@ -13,11 +14,14 @@ One of the following methods can be used to setup/create a primary key.

## Conventions

By convention, a property named `Id` or `<type name>Id` will be configured as the key of an entity.
By default, a property named `Id` or `<type name>Id` will be configured as the key of an entity.

[!code-csharp[Main](../../../samples/core/Modeling/Conventions/KeyId.cs?name=KeyId&highlight=3)]

[!code-csharp[Main](../../../samples/core/Modeling/Conventions/KeyTypeNameId.cs?name=KeyIdhighlight=3)]
[!code-csharp[Main](../../../samples/core/Modeling/Conventions/KeyTypeNameId.cs?name=KeyId&highlight=3)]

> [!NOTE]
> [Owned entity types](xref:core/modeling/owned-entities) use different rules to define keys.

## Data Annotations

Expand All @@ -34,3 +38,12 @@ You can use the Fluent API to configure a single property to be the key of an en
You can also use the Fluent API to configure multiple properties to be the key of an entity (known as a composite key). Composite keys can only be configured using the Fluent API - conventions will never setup a composite key and you can not use Data Annotations to configure one.

[!code-csharp[Main](../../../samples/core/Modeling/FluentAPI/KeyComposite.cs?highlight=11,12)]

## Key types and values

EF Core supports using properties of any primitive type as the primary key, including `string`, `Guid`, `byte[]` and others. But not all databases support them. In some cases the key values can be converted to a supported type automatically, otherwise the conversion should be [specified manually](xref:core/modeling/value-conversions).

Key properties must always have a non-default value when adding a new entity to the context, but some types will be [generated by the database](xref:core/modeling/generated-properties). In that case EF will try to generate a temporary value when the entity is added for tracking purposes. After [SaveChanges](/dotnet/api/Microsoft.EntityFrameworkCore.DbContext.SaveChanges) is called the temporary value will be replaced by the value generated by the database.

> [!Important]
> If a key property has value generated by the database and a non-default value is specified when an entity is added then EF will assume that the entity already exists in the database and will try to update it instead of inserting a new one. To avoid this turn off value generation or see [how to specify explicit values for generated properties](../saving/explicit-values-generated-properties.md).