Building Data-Driven Web Apps with Flask and SQLAlchemy Transcripts
Chapter: Modeling data with SQLAlchemy classes
Lecture: Indexes
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Let's return to our package class that defines
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the table and the database.
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I said there was a few other things that we needed to do
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and well, let's look at a few of the problems
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and then how to fix them.
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created_date. created_date is great.
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What is it's value if we forget to set it?
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Null. That's not good.
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We could say it's required and make us set it
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but wouldn't it even be better if SQLAlchemy itself
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would just go right now, the time is now
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and that's the time it's created on first insert?
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Super easy to do that.
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But, we've got to explicitly say that here.
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We're here in the emails.
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Maybe we want to be able to search by author/email.
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So, we might want to have an index here so we can ask
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quickly ask the question: show me all the packages authored
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by the person with such and such email. Boom.
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If we had a index, this could be super, super fast.
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The difference in in terms of query time for a query
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with an index and without an index with a huge amount
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of data can be like a thousand time slower
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it's an insane performance increase to have an index.
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Helps you sort faster, it helps you query faster
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and so on.
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So, we're going to want to add an index on some of these.
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And maybe the summary is required, but the description
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is not required.
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So, we would like to express that here as well.
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So, SQLAlchemy has capabilities for all these.
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So, let's start with the default value.
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So, pretty easy, we're going to set defaults here
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and it could be something like 0 or True or False
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if that made sense, it doesn't for datetime.
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Well, what would be the value for a datetime?
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Well, let's use the datetime module and we can import
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that at the top.
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And use datetime.now.
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Now, notice if I just press end and then enter
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bicharm is super helpful and it puts parenthesis here.
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That is a horrible thing that happened.
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What this will do is take the current time
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when the application starts and say well that time
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is the default value. No no.
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That's not what we want.
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What we want, is we want this function now to be passed off
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in any time SQLAlchemy is going to put something
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in the database, it goes, oh, I have a default value
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which is a function so let me call that now
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to get the value.
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So, that will do the trick of getting the insert time
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exactly as you want.
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Here we can say, nullable=False
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you can say, nullable=True.
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Not all databases support the concept of nullability
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like, I think SQLite doesn't, but you don't want to
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necessarily guarantee that you're always working
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with that, right?
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We may also want to say, all like, all the packages
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or the top ten packages are the most recent ones.
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And, for that you might want an index cause then that
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makes it really fast to do that sort, so we can say
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index=True.
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And that's all you got to do to add an index
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it's incredible.
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We also may want to ask, you know, uh, show me all
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the packages this person has written.
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So, then we'd say index is true, that'll be super fast.
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Also, you might ask, what are all the packages
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or even, how many of them are there that have say the
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MIT license?
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And, then you could do a count on it or something, right?
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So, this index will make that query fast.
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These we'll deal with when we get to relationships.
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But, these simple little changes here will make it
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much, much better and this is really what we wanted
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to define in the beginning, but I didn't want
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to overwhelm you with all the stuff at the start with.
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All right, so we have our database over here.
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Let's go and you know, and we have it here as well.
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Let's go and run the app, rerun it
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see if everything's working and if we just refresh this
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what's going to happen. Uh, sad face, nothing happened.
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Where are my indexes?
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Where is my null, well, nullability statements
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things like that.
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This is a problem, this is definitely a problem.
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The reason we don't see any changes here is that SQLite
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will not create or modifications to a table.
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It'll create new ones, great new tables.
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If I add a new table it'll create it like uh
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release it or something, you would see it show up
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when I ran it. But, once it exists, done.
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No changes, it could lose data or it could make other
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problems, so it's not going to do it.
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Leader, and this is very common, you want this
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but SQLAlchemy won't do it.
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We're going to later talk about something called
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Alembic which will do database migrations
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and evolve your database schema using our classes here.
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But, we're not there yet.
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We just are trying to get SQLAlchemy going.
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So, for now, what do we do, how do we solve this?
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We could either just delete this file, drop that table
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and just let it recreate it, right, we don't really
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have any data yet.
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When you get to production migrations, but for now
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just super quick, let's just drop that table.
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I'll rerun the app, refresh the schema, expando
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and look at that, here's our indexes and author/email
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creator date and license, here's our uniqueness
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constraint on the id which comes with part
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of the primary key.
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You can see those, uh, blue things right there
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those are the indexes.
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So, we come over here and modify the table, you can see
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here's your indexes and your keys and so on. Cool, huh?
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All we need to do is put the extra pieces of information
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on here and when we enter one of these packages
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we'll get in and out and we'll have an index for it
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and so on. Super, super cool.
