Building Data-Driven Web Apps with Flask and SQLAlchemy Transcripts
Chapter: Client and server-side validation
Lecture: Viewmodel base class
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The key thing to wrap your mind around
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as your work with these view models
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is that the data exchange is deeply tied
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to the HTML template, and what this
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view method has to pass to it.
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It's going to be solely the job
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of the view model to know how that data
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is exchanged, how it is being passed back
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and what kind of validation we need.
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So it's all about this data exchange and validation.
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What you'll see is that every view model
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is different and it's in particular tied to
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this template and this method.
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However, there's certain common functionality
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that we have to use, like every single template
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has to have this for the shared layout.
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We'll also see that most templates
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and anything that has a form really
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has to have some kind of error message
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like this as well.
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So, these kind of things will be great
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to put into a base class along with
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some of the behaviors
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that every view model's going to need.
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So let's go and do that real quick here.
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Create a folder called viewmodels.
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Because very much like our templates here
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we have some organization, like this account
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has these three views, right?
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CMS style also has an index
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but that's not the same as an accounts index, right?
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Very, very different.
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This organization equally applies
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almost exactly over here because remember
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the view models are tied to those templates
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with that very same organization.
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So let's go over here and create shared one
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and over here and create an account one.
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So the shared is where we're going
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to put our base class.
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Called ViewModelBase.
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ViewModelBase, like so
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with no, nothing derive from it.
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And it's going to use some stuff
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from Flask as we'll see.
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So what we're going to do is have
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an initializer, a little constructor here
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and in here we're going to store the request.
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So we're going to get the request
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because we need to do certain things with it
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and we could also define this as a request
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from Flask, so that when we say
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self.request. we get
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all sorts of good stuff there, all right.
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The other thing we might want to have
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is we might want to have a request dict
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or something like that.
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Remember we had for this request
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dict library that we created
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that will unify where all the data
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is coming from so we want to take
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that and say create and we're going
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to give it a default value.
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I guess could be like this.
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And we don't have any route args here
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not yet anyway. So that's going to go
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and create our dictionary here.
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And then we also have some common data
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we had our error which is an optional
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from typing, Optional string
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that's equal to None.
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We have our user_id which is an optional.
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And this one we can go to our cookie_auth.
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Here we're going to say
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get_user_id_via_auth_cookie
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what's it going to take, it's going
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to take self a request.
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Perfect, so that's going to give us
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some back, either the user, or not.
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And then we'll see that
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classes can derive from this ViewModelBase
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and they can add additional fields.
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This is great but, what was happening over here?
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We were creating a dictionary and sending it back.
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Well, we're going to want to be able to
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convert from this class
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from an object, into a dictionary.
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So let's add a little function here as well.
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To dict and it's simply going to return
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the simplest possible thing for now.
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It's just going to return the dictionary
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that backs the storage for all the fields in the class
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not for the view model base
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but for the instance of whatever it is.
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So a extra view model, or a package view model
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whatever the things are that we're going to create.
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So here's going to be our base class
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and this is going to provide a lot of functionality
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for working with this data exchange
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and the validation and so on.
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However, to really make it useful we have to
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create individual, specific ones tied to these views
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by creating classes derived from our ViewModelBase.
