Now that the data needed to create a `BroadcastArea` is pretty
lightweight because it doesn’t include the GeoJSON we can go back to
putting it in memory when we start up the app, to make the pages load
really fast.
Rough estimate for the size of this dataset:
> 10,000 areas
> Average length of area name = 20 characters
> Average length of area id = 20 characters
> Size of one area in bytes = 20 + 20 = 40
> Size of dataset = 40 * 10,000 = 400,000 bytes = 400kb
I think that even with good indexes, querying the area names from one
table is always going to be slow because there’s so much GeoJSON to scan
past.
This commit splits the data into two tables, one for the names and
grouping IDs and one for the blobs of GeoJSON. So for most pages the app
will never even be looking at the table where the GeoJSON is held.
I don’t know if this is a proper, normalised way of structuring the
data, but it does go brrr.
Rather than querying all the features whenever we look up area(s) let’s
only get them when we need them.
The features are really big blobs of data to pass around, so there’s a
significant performance gain to be had from doing this.
If a library has groups, we should show a link instead of selecting the
group directly.
Then we can give the user the choice of selecting the whole of that
group, or specific areas within the group.
For now the only libraries we have with groups are local authorities,
which group electoral wards.
Password managers will try to guess what they should save as a username
by looking at the fields on the page where you set up your password.
When registering from an invite the email address (what we use as a
username) is predefined, and only shown on the page as text, not an
input.
This commit also adds a hidden input field for password managers to pick
up.
Adapted from: https://github.com/UKGovernmentBEIS/beis-opss-psd/blob/master/app/views/users/complete_registration.html.erb#L29-L36
This commit adds a new model class which can be used by any app to
interact with a broadcast area. A broadcast area is one or more polygons
representing geographical areas.
It also adds some models that make browsing collections of these areas
more straightforward. So the hierarchy looks like:
> **BroadcastAreaLibraries*
> Contains multiple libraries of broadcast area
> > **BroadcastAreaLibrary**
> > A collection of geographic areas, all of the same type, for example
> > counties or electoral wards
> > **BroadcastArea**
> > Contains one or more shapes that make up an area, for example
> > England
> > > **BroadcastArea.polygons[n]**
> > > A single shape, for example the Isle of Wight or Lindisfarne
> > > > **BroadcastArea.polygons[n][o]**
> > > > A single coordinate along a polygons
The classes support iteration, so all the areas in a library can be
looped over, for example if `countries` is an instance of
`BroadcastAreaLibrary` you can do:
```python
for country in countries:
print(country.name)
```
The `BroadcastAreaLibraries` class also provides some useful methods for
quickly getting the polygons for an area or areas, for example to
render them on a map. So if `libraries` is an instance of
`BroadcastAreaLibraries` you can do:
```python
libraries.get_polygons_for_areas_long_lat('england', 'wales')
```
This will give polygons for the Welsh mainland, the Isle of Wight,
Anglesey, etc.
The models load data from GeoJSON files, which is an open standard for
serialising geographic data. I’ve added a few example files taken from
http://geoportal.statistics.gov.uk to show how it works.
