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- [Instructor] Hi and
welcome back to the course.

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In this video and the next,
we're going to go over

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how easy it is

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to add more to our API
and make it more powerful.

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We're going to do that by
adding the concept of stores,

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in this video.

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Let's go over to our models folder,

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because we're going to start
by creating a new model

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as opposed to a new resource.

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That is because the resource
normally has to use the model

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to extract data from the database,

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the model kind of stands on itself.

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Let's right click the models folder

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and create

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a store

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.py file

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and what we're gonna put in the store.py

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is a copy of the item model,
so I'm just gonna select

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everything there, copy it
and paste it in the store.py.

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Now, of course we're gonna
change a few things here.

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We're gonna call this StoreModel,

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the table name is going to be stores,

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it's not gonna have a price column,

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it's just gonna have an id and a name.

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The JSON method is going
to be slightly different

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and that is going to be,
instead of name and price

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it's going to be name and items

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and here, we're going to put

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self.items

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and we're gonna get into exactly

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how we get those items in a moment.

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Okay?

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The find_by_name is going
to be exactly the same,

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we're going to be able to
retrieve a store by its name

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and then we're gonna be
able to save to the database

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and delete it from the database as well.

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So, nothing there is going to change.

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Now I have to make sure
that we have this self.items

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thing.

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Let's go to the item model, first,

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and we're going to add something there.

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What we're going to add
here is a new column

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on the item.

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This column is going to be store_id

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and this is going to be an integer

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because it has to match
exactly the store's id type

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and it's also gonna have something else,

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which is db.foreignkey

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and the foreign key

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is going to be stores,

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which is the table name of the store,

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.id, which is the column name.

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Okay, so, what does this mean?

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What is a foreign key

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and why are we putting a store id here?

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Well, let me show you
a small diagram, here.

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Imagine that we have, well, we do have,

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these two models: the
item and the store model.

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If we create, say, three items

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and they all have the same
store id, which is two,

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and then we have a store
that has the id two

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we can link them

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in our minds

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by saying that these three
items belong inside this store

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or they are somehow
linked and that is because

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we have the item, we have the store id

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and therefore we can
then go to the database

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and find the store that has this id.

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Similarly, if we have the store,

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we can go to the items table and find out

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which items have the same store id

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as this store's id.

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So, we have the store model,

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we can find all the
items that belong to it

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and if we have an item
we can find out the store

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that it belongs to.

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So, they are somehow linked and this link

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is caused or created by the foreign key.

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The id in the store is a primary key

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and the store id in the
item is a foreign key

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because it has values

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identical to the id values

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of another table.

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In this scenario, here,
where we have three items

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with store id two and a store with id two,

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we would not be able to delete this store.

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The relational database engine,

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such as SQL Lite or Postgres or My SQL

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would not let you delete this store

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because there are foreign key references.

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So, you'd first have to delete these items

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or move them somewhere
else, change the store id

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before you can delete this store.

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So, it allows us some degree
of control or security

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and in knowing that these two are linked

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and it's not just

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in our heads

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and so by using a foreign key
that gives us that security

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that we can't delete things, for example.

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Okay?

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So, this is why we're adding
this store id to the model

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because then we're going to be able to say

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what store do we belong
to and we're gonna be able

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to find the store.

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But there would be a
way to do that with SQL,

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with a join and we're going
to look at what a join is

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in a future video

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but we don't have to use joins

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because SQLAlchemy does that for us.

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All that we have to do is, now,

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say store

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is

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db.relationship

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storemodel and voila.

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If you ever used joins in SQL in the past,

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you'll be pleased to
know that you no longer

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have to do that.

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What this does now is that it sees

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that we have a store id

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and therefore we can find

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a store in the database
that matches this store id.

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So, now, every item model
has a property store

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that is the store which
matches this store id

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in its id.

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Hopefully, all of that makes sense

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but also we can do a back reference.

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A back reference does the opposite.

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It allows a store to see which items

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are in the items database,
or in the items table,

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with the store id equal to its own id.

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The way we do that is
items equal db.relationship

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itemmodel.

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How easy is that?

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Okay, so what that is doing it it's saying

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we have a relationship with itemmodel

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and then SQLAlchemy says, okay,
there's a relationship there

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what's the relationship?

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And then it goes into the item

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and it finds that there's a store id here

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and then it goes, ah-ha,
there's a store id in the item

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which means

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that one item is related to a store.

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Therefore there could
be more than one item

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related to the same store.

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There could be two items
related to the store,

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like in this example or three
items related to the store.

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So it knows

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that the store and the item,

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this variable here, is a single store,

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i.e.: there's only one store
that and item is related to

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but the items variable in the store

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can be many items, so, therefore,

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if we go to the store this variable here

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is a list of item models
because it knows that it is a

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many-to-one relationship.

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There could be many items
with the same store id.

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So, hopefully all that makes sense

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and now we can go back
here into self.items

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and we can access

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each of the item JSONs.

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What we're gonna do is,
as you know already,

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we're gonna replicate what
we have in our item list

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get endpoint.

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Item.json

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for item in

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self.items

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and that's it.

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Now, there's an important thing here

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that often goes overlooked and that is

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that there is a relationship
that has been created

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as soon as we create a store model.

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So, if we have many stores
and we have many items

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whenever we create a store model,

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we're going to go and create
an object for each item

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in a database that matches that store id.

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Now, if we have very
few items, that's fine

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but if we have a lot of
items that can be a really

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expensive operation.

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So, what we can do is
we can tell SQLAlchemy

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to not do that,

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do not go into the items table

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and create an object for each item yet.

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The way we do that is we say

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lazy is dynamic

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and now whenever we access the JSON method

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we're gonna get an error

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unless

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we use

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.all.

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So what's the key take, here?

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When we use lazy equals dynamic,

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self.items no longer is a list of items,

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now it is a query builder
that has the ability

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to look into the items table,

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then we can use .all to
retrieve all of the items

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in that table.

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Which means that until
we call the JSON method

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we are not looking into the table,

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which means that creating
stores is very simple.

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However, it also means that every time

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we call the JSON method

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we have to go into the table,

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so then it is gonna be slower
and what I mean with that is

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we create a store so we
load up all the items

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and then we can call the
JSON method many times

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for free, essentially.

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If we use lazy dynamic, every
time we call the JSON method

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we have to go into the table,

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so then that is slower.

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So, there's a trade-off there
between speed of creation

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of the store and speed of
calling the JSON method

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that you have to think about
which one is more important.

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In our case, because the
store model gets created

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when we wanna access the
data, it's gonna be like that

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in our resource, I'm gonna stick with

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this way of doing things but,
just food for thought, there,

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something you have to
take into consideration.

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Of course, something else we have to do

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is go to the item model and
make sure that we're accepting

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a store id there as well

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and saving that to the database.

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What that'll do is that'll
make sure that the store id

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for the specific item model we're creating

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gets assigned the value that we pass in

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and that that gets put in the database.

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Now that we're here, let's
go into the resources

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and just make sure that
we're accepting this as well

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and we don't forget later.

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I'm gonna copy the add argument

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and paste it in, there.

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Rename it to store id,

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change the type to int

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and change the help message to something

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a bit more helpful, like
every item needs a store id.

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Now, something else to remember

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is that whenever we create an item model

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we're gonna have to pass in the store id.

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So, for example, here in the post,

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we are creating a new item,

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the data that is parsed
from the parse args method

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is now going to contain a store
id that we need to pass in

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and similarly, down here in the put method

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we have to make sure to include

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data

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store id.

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I'll leave it up to you
to change the store id

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in here, if you want as
well, which you can do.

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Okay?

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Also remember that data
price and data store id

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can be simplified to

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data

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that is just going to
exactly the same thing.

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It's going to pass in price as data price

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and store id as data store id,

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so that we can do that,
if you find that easier.

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Okay?

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So, that's everything for this video.

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Now we've created the store model

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and we've linked it up with the item.

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In the next video we're going
to create the store resource,

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so, I'll see you there.