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Let's look at updating entities. Specifically, updating ComicBook entities in the Repository class' `UpdateComicBook` method.
Follow Along
To follow along commiting your changes to this course, you'll need to fork the dotnet-comic-book-gallery-model repo. Then you can clone, commit, and push your changes to your fork like this:
git clone <your-fork>
cd dotnet-comic-book-gallery-model
git checkout tags/v5.3 -b updating-entities
Let's look at updating entities,
specifically updating
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comic book entities in the repository
classes UpdateComicBook method.
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As mentioned in the previous video,
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EF tracks the changes to entities
that are loaded into the context.
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The UpdateComicBook method is past
an instance of a comic book entity, but
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that entity is not being
tracked by the context.
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Entities that are not being tracked by
the context are said to be disconnected or
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detached.
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There are a number of ways that we can
persist data changes contained within
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disconnected or detached entities.
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Our first option is to retrieve
the entity from the database.
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Then write code to manually
update the property values.
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To retrieve the entity,
we can use the DB set find method.
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Don't forget, we need to start with
getting an instance of the context.
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Using (Context context
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= GetContext()).
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Then we can retrieve the comic book,
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ComicBook comicBookToUpdate=
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Context.ComicBooks.Find(comicBook.Id).
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Then we simply update the retrieved entity
property values with the values from
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the passed in entity.
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comicBookToUpdate.SeriesId =
comicBook.seriesId.
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comicBookToUpdate.IssueNumber =
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comicBook.IssueNumber.
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comicBookToUpdate.Descriptions =
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comicBook.Description.
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comicBookToUpdate.PublishedOn =
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comicBook.PublishedOn.
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And then comicBookToUpdate.AverageRating
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= comicBook.AverageRating, and
call the SaveChanges method.
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When we call the SaveChanges method,
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EF will detect the changes to the entity
that is being tracked in the context.
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Generate a sequel update statement and
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execute that statement
against the database.
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All this approach is simple and
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easy to understand,
it's tedious to write and maintain.
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Let's look at another option.
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In this option,
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we still use the DB set find method to
retrieve the entity from the database.
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But instead of manually setting
the new property values,
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we retrieve the entity's
entry from the context.
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And call the current value's set values
method to copy the passed in entity's
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property values to the entry's
current values collection.
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context.Entry(ComicBookToUpdate).CurrentV-
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alues.SetValues, then pass
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in the (comicBook) parameter.
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After calling the sub values method,
the entity state assuming that at least
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one property value is updated,
will be set to modified.
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A state of modified indicates
that the entity has a change
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that hasn't been persisted
to the database.
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When we call the SaveChanges method, EF
will generate a SQL update statement for
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the modified entity and execute that
statement against the database.
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Let's test our changes.
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Set a break point just after the call to
the save changes method and run the app.
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Enter 3 to view the detail for
issue number 3 of the bone series.
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Then enter U to update the comic book.
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Let's change the average rating.
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Enter 5 to select a property,
then enter 7 for the value.
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Enter S to save the comic book.
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Now, we're at our break point.
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The save changes method was just called.
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In the output window, we can see the
queries that were generated and executed.
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The first query was the query to
retrieve the comic book entity.
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And the second query was to
update the average rating column.
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This option is an improvement
over the first.
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And that we don't have to manually
update the entity properties.
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A downside to both approaches is
that we have to retrieve the entity
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from the database.
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Which means that updating
an entity requires two queries.
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One to retrieve the entity and
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another to persist any changes
to the entity to the database.
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Let's look at an option that
doesn't require two queries.
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Instead of retrieving
the entity from the database,
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we can attach the passed-in entity to the
context using the DB set attach method.
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context.ComicBooks.Attach(comicBook).
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After attaching the entity to the context,
its state will be set to unchanged.
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EF is unable to detect whether or
not any of the entity's property values
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are different from the values that
are currently in the database.
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We can force EF to treat the entity's values as new
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values by setting the associated
entry state to modified.
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context.Entry(comicBook).State =
EntityState.Modified.
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Now, when we call the SaveChanges method,
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EF will persist each of the entity's
property values to the database.
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Let's test our changes again.
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Enter 3 to view the detail for
issue number 3 of the bone series.
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Then enter U to update the comic book.
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Let's change the description.
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Enter 3 to select the property.
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This is my new description.
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Then enter S to save the comic book.
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We're at our break point again,
just after the SaveChanges method call.
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In the output window, we can see the query
that was generated and executed.
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So we were able to eliminate the query to
retrieve the entity from the database.
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But on the downside,
as we can see in the generated query,
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every column value is being
updated in the database.
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Not just the ones that
have actually changed.
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In most situations,
this approach works fine.
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Though, there might be times that an
entity contains a property that shouldn't
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ever be updated.
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Let's imagine that once a comic book
has been added to the database,
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its issue number shouldn't be changed.
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Let's start with rewriting
this line of code, so
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that we retrieve the current
books entry into a variable.
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Var comicBookEntry =
context.Entry(comicBook).
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Then, comicBookEntry.State
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= EntityState.Modified.
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Then use the entries property method
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to get a reference to the issued
number properties entry object.
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And set the entries is
modified property to false.
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comicBookEntry.Property("IssueNumber").Is-
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Modified = false;.
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This line of code will prevent the
IssueNumber property from being included
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in the generated SQL update query.
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Go ahead and remove or comment out this line of code.
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We want our users to be able to
change a comicBook's issue number.
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After a code challenge,
we'll see how to delete entities
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