Documenting entity key mapping configuration

Author: SteveWilkes

docs/src/Entity-Mapping.md

@@ -2,9 +2,9 @@ Mapping to entities - an object with a member decorated with a [`KeyAttribute`](
 
 ### Entity Keys
 
-Members decorated with a `KeyAttribute` are not mapped, unless the mapping is a deep clone. Entity key values are usually generated by a data store, and some ORMs will throw an exception if a key value is updated in code.
+By default, members decorated with a `KeyAttribute` are not mapped, unless the mapping is a deep clone. Entity key values are usually generated by a data store, and some ORMs will ignore key value updates, or in some cases, throw an exception.
 
-If you need to map to a `KeyAttribute` member, you can override this behaviour by configuring a [custom data source](/configuration/Member-Values).
+If you need to map `KeyAttribute` members, you can [configure the behaviour](/configuration/Entity-Mapping) that suits your needs, or configure a [custom data source](/configuration/Member-Values).
 
 ### Optional Related Entity Keys
 

docs/src/configuration/Entity-Mapping.md

@@ -0,0 +1,102 @@
+By default, members decorated with a `KeyAttribute` are not mapped, unless the mapping is a deep clone. Entity key values are usually generated by a data store, and some ORMs will ignore key value updates, or in some cases, throw an exception.
+
+```cs
+// Retrieve the existing Customer entity:
+var customer = await dbContext
+    .Customers
+    .FirstOrDefaultAsync(c => c.Id == customerDto.Id);
+
+if (customer == null)
+{
+    // Deal with Customer not found
+}
+
+// Update the Customer from the DTO - this does not map Customer.Id:
+Mapper.Map(customerDto).Over(customer);
+
+// Save the changes - no problems!
+await dbContext.SaveChangesAsync();
+```
+
+There are circumstances where you might want to map entity keys, though. A typical case might be to [delete an entity by id](https://stackoverflow.com/questions/2471433/how-to-delete-an-object-by-id-with-entity-framework):
+
+```cs
+// Create a dummy Customer entity from the DTO:
+var customerToDelete = Mapper.Map(customerDto).ToANew<Customer>();
+
+// Attach then remove the Customer from the context:
+dbContext.Customers.Attach(customer);
+dbContext.Customers.Remove(customer);
+
+// Save the changes - no Customer.Id, so does not work!
+await dbContext.SaveChangesAsync();
+```
+
+### Mapping Keys
+
+There's a number of options to change entity key mapping behaviour, depending on your needs.
+
+To map keys by default **in all mappings**, use:
+
+```cs
+Mapper.WhenMapping.MapEntityKeys();
+
+// - - - - - - - - - - - - - - - 
+
+// Customer.Id will now be mapped:
+var customerToDelete = Mapper.Map(customerDto).ToANew<Customer>();
+```
+
+To map keys by default when mapping **between two particular types**, use:
+
+```cs
+Mapper.WhenMapping
+    .From<CustomerDto>() // Apply to CustomerDto mappings
+    .ToANew<Customer>()  // Apply to Customer creations
+    .MapEntityKeys();    // Map entity keys
+
+// - - - - - - - - - - - - - - - 
+
+// Customer.Id will now be mapped:
+var customerToDelete = Mapper.Map(customerDto).ToANew<Customer>();
+```
+
+To map keys for an **individual mapping**, use:
+
+```cs
+// Map the DTO, including to Customer.Id:
+var customerToDelete = Mapper
+    .Map(customerDto)
+    .ToANew<Customer>(cfg => cfg.MapEntityKeys());
+```
+
+## Opting Out
+
+If the default behaviour is changed to map keys, key mapping can be disabled when necessary.
+
+To ignore keys when mapping **between two particular types**, use:
+
+```cs
+// Map keys by default:
+Mapper.WhenMapping.MapEntityKeys();
+
+// Ignore entity keys when mapping from any
+// source Type to a Customer:
+Mapper.WhenMapping
+    .To<Customer>()      // Apply to Customer mappings
+    .IgnoreEntityKeys(); // Do not map entity keys
+```
+
+To ignore keys for an **individual mapping**, use:
+
+```cs
+// Map keys by default:
+Mapper.WhenMapping.MapEntityKeys();
+
+// - - - - - - - - - - - - - - - 
+
+// Ignore entity keys for this mapping:
+Mapper
+    .Map(customerDto)
+    .Over(customer, cfg => cfg.IgnoreEntityKeys());
+```

mkdocs.yml

@@ -45,6 +45,7 @@ nav:
         - Dependency injection: configuration/Dependency-Injection.md
         - Member values: configuration/Member-Values.md
         - Constructor arguments: configuration/Constructor-Arguments.md
+        - Entity mapping: configuration/Entity-Mapping.md
         - Member name patterns: configuration/Member-Name-Patterns.md
         - To-string formatting: configuration/To-String-Formatting.md
         - Enum pairing: Enum-Mapping/#configuring-enum-pairs