text stringlengths 1 81 | start float64 0 10.1k | duration float64 0 24.9 |
|---|---|---|
First gets me that first flight. | 3,444.48 | 2.44 |
And so now, I have this flight
from New York to London. | 3,446.92 | 3.5 |
And just as in any Python
object, I can begin | 3,450.42 | 2.13 |
to access properties of that object. | 3,452.55 | 2.18 |
I can say, all right,
flight, what is your ID? | 3,454.73 | 2.26 |
Flight, what is your origin? | 3,456.99 | 1.77 |
Flight, what is your destination? | 3,458.76 | 1.44 |
Flight, what is your duration? | 3,460.2 | 1.63 |
And I can access, as values, all
of the properties of this flight | 3,461.83 | 3.54 |
that I ultimately care about. | 3,465.37 | 1.22 |
And if I want to delete the flight, I
can say something like flight.delete. | 3,466.59 | 5.13 |
Now, ultimately though,
this is not the model | 3,471.72 | 2.79 |
that I actually want
to represent my flight. | 3,474.51 | 2.1 |
Because here again, I'm using a
character field, a char field, | 3,476.61 | 3.48 |
for things like origin and destination. | 3,480.09 | 1.92 |
When in reality, I'd probably like
to use something like another table | 3,482.01 | 4.38 |
for representing airports,
and then some relationship | 3,486.39 | 3.36 |
between every flight and an airport. | 3,489.75 | 2.34 |
So let's go ahead and try
and implement that idea now, | 3,492.09 | 2.46 |
that I can go back into
models.py and create a new class. | 3,494.55 | 4.29 |
I'll create a class called airport. | 3,498.84 | 2.4 |
That is also a model. | 3,501.24 | 1.77 |
And I'd like for this
airport class to have | 3,503.01 | 2.52 |
a code, which is a character
field with a max length of 3, | 3,505.53 | 5.43 |
for the airports code. | 3,510.96 | 1.47 |
As well as a city, which
would be a character field | 3,512.43 | 2.49 |
with a max length of 64. | 3,514.92 | 2.79 |
And let's also give this
airport a string representation. | 3,517.71 | 3.79 |
We'll say that the string
representation of an airport | 3,521.5 | 2.39 |
will just be the city of the
airport, and then in parentheses, | 3,523.89 | 4.29 |
the code of the airport. | 3,528.18 | 1.1 |
So it will be something like New
York, and then in parentheses | 3,529.28 | 2.583 |
JFK to represent a particular airport. | 3,531.863 | 4.147 |
And now, our flight model
needs to change a little bit. | 3,536.01 | 3.27 |
No longer will origin and
destination be character fields | 3,539.28 | 3.27 |
that are just storing text. | 3,542.55 | 1.53 |
But instead, origin is
going to be a foreign key. | 3,544.08 | 4.83 |
A foreign key that references another
table, like the airport table. | 3,548.91 | 5.157 |
And then, I can provide
some additional arguments. | 3,554.067 | 2.083 |
So this alone would be enough. | 3,556.15 | 1.35 |
But I can add some additional arguments
like on delete equals models.cascade. | 3,557.5 | 5.57 |
So what does this mean? | 3,563.07 | 1.21 |
Well, when I have tables that
are related to each other, | 3,564.28 | 3.052 |
SQL needs some way of
knowing what should | 3,567.332 | 1.708 |
happen if you ever delete something. | 3,569.04 | 1.83 |
If I have a flight from JFK
to London, and later in time | 3,570.87 | 5.07 |
decide to delete JFK
airport from my database, | 3,575.94 | 2.79 |
what should happen to that flight? | 3,578.73 | 1.65 |
What happens to flights when the thing
that it is referencing gets deleted? | 3,580.38 | 4.44 |
What models.cascade means
is if I were to ever delete | 3,584.82 | 2.94 |
an airport from the airports
table, it's going to also delete | 3,587.76 | 3.54 |
any of the corresponding flights. | 3,591.3 | 1.737 |
And there are other on
delete parameters you | 3,593.037 | 1.833 |
can set for saying like, don't
even let me delete an airport | 3,594.87 | 2.57 |
if there are flights
that are leaving from | 3,597.44 | 1.75 |
or going to that airport,
that's called models.protect. | 3,599.19 | 3.06 |
But there are other ways of implementing
similar types of constraints. | 3,602.25 | 4.41 |
And the other argument
that I'm going to provide | 3,606.66 | 2.04 |
is what's called a related name. | 3,608.7 | 2.49 |
And a related name, as
we'll see in just a moment, | 3,611.19 | 2.34 |
is going to be a way of me accessing
a relationship in the reverse order. | 3,613.53 | 4.47 |
That from a flight, I can take
a flight and say .origin to get | 3,618 | 4.17 |
the flight's origin in airport. | 3,622.17 | 1.648 |
But the other question I might want
to ask is in the reverse order. | 3,623.818 | 2.792 |
If I have an airport, how do
I get all of the flights that | 3,626.61 | 3.6 |
have that airport as an origin? | 3,630.21 | 1.83 |
And so here, if I give a related
name to this foreign key, | 3,632.04 | 3.24 |
Django will automatically
set up the relationship | 3,635.28 | 2.73 |
going in that opposite direction. | 3,638.01 | 1.99 |
And so here, well, if
we have an airport, | 3,640 | 2.39 |
and I want to know all of the flights
that have that airport as their origin, | 3,642.39 | 3.36 |
the reasonable name for a related name
here is something like departures. | 3,645.75 | 4.68 |
So if I have an airport,
I can access all | 3,650.43 | 2.61 |
of the departures, which gets
me all of the flights that | 3,653.04 | 2.91 |
are leaving from that airport. | 3,655.95 | 2.4 |
And I'll likewise do the same
thing here for destination. | 3,658.35 | 2.61 |
Instead of a character field
it's going to be a foreign key. | 3,660.96 | 3.12 |
It's going to reference airport. | 3,664.08 | 1.92 |
When we delete it, we'll
go ahead and cascade it. | 3,666 | 2.31 |
And the related name will be arrivals. | 3,668.31 | 2.85 |
Because if I have an
airport, I might want | 3,671.16 | 1.89 |
to access all of the
arrivals, all of the flights | 3,673.05 | 2.76 |
that correspond to
flights that are arriving | 3,675.81 | 2.91 |
at that particular destination. | 3,678.72 | 2.675 |
And so now, I've done two things. | 3,681.395 | 1.375 |
I've added a new class
called airport, and I've | 3,682.77 | 2.85 |
modified my existing flight model. | 3,685.62 | 2.76 |
So this has changed in my
Python code, but it hasn't yet | 3,688.38 | 2.73 |
changed in my database. | 3,691.11 | 1.78 |
So in order to make the change in the
database, again, it's a 2-step process. | 3,692.89 | 3.78 |
Step one, python
manage.py, make migrations, | 3,696.67 | 3.83 |
to say look for any new changes
that have been made to models.py, | 3,700.5 | 4.87 |
and go ahead and create
a migration instruction | 3,705.37 | 2.96 |
for how to make those
changes to the database. | 3,708.33 | 2.62 |
And here, we see that we've
created a new migration file. | 3,710.95 | 4.49 |
And this migration is going to
create a model called airport. | 3,715.44 | 3.09 |
And it's also going to
alter the destination field | 3,718.53 | 2.85 |
and alter the origin
field on my flight model. | 3,721.38 | 2.91 |
Because as we know, we've
changed destination and origin | 3,724.29 | 3.06 |
to no longer be character
fields, but to instead | 3,727.35 | 2.43 |
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