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| post | A Tour of myPrayerJournal: The Data Store | 2018-08-30 12:15:00 | Daniel |
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NOTES:
- This is post 6 in a series; see the introduction for all of them, and the requirements for which this software was built.
- Links that start with the text "mpj:" are links to the 1.0.0 tag (1.0 release) of myPrayerJournal, unless otherwise noted.
Up to this point in our tour, we've talked about data a good bit, but it has all been in the context of whatever else we were discussing. Let's dig into the data structure a bit, to see how our information is persisted and retrieved.
Conceptual Design
The initial thought was to create a document store with one document type, the request. The request would have an ID, the ID of the user who created it, and an array of updates/records. Through the initial phases of development, our preferred document database (RethinkDB) was going through a tough period, with their company shutting down; thankfully, they're now part of the Linux Foundation, so they're still around. RethinkDB supports calculated fields in documents, so the plan was to have a few of those to keep us from having to retrieve or search through the array of updates.
We also considered a similar design using PostgreSQL's native JSON support. While it does not natively support calculated fields, a creative set of indexes could also suffice. As we thought it through a little more, though, this seemed to be over-engineering; this isn't unstructured data, and PostgreSQL handles max-length character fields very well. (This is supposed to be a "minimalist" application, right?) A relational structure would fit our needs quite nicely.
The starting design, then, used 2 tables. request had an ID and a user ID; history had the request ID, an "as of" date, a status (created, updated, etc.), and the optional text associated with that update. Early in development, the journal view brought together the request/user IDs along with the latest history entry that affected the text of the request, as well as the last date/time an action had occurred on the request. When the notes capability was added, it got its own note table, with a structure similar to the history table, but without a status, and the text is not optional. As snoozing and recurrence capabilities were added, those fields were added to the request table (and the journal view).
The final design uses 3 tables, 2 of which have a one-to-many relationship with the third; and 1 view, which provides the calculated fields we were going to have RethinkDB calculate for us.
Database Changes (Migrations)
As we ended up using 3 different server environments over the course of this project, we ended up writing a DbContext class based on our existing structure. For the Node.js backend, we created a DDL file ([mpj:ddl.js][ddl.js], v0.8.4+) that checked for the existence of each table and view, and also had the SQL to execute if the check failed. For the Go version (mpj:data.go, v0.9.6+), the EnsureDB function does a similar thing; looking at line 347, it is checking for a specific column in the request table, and running the ALTER TABLE statement to add it if it isn't there.
The only change that was required since the F#/Giraffe backend has been in place was the one to support request recurrence. Since we did not end up with a scaffolded EF Core initial migration/model, we simply wrote a SQL script to accomplish these changes (mpj:sql directory).1
The EF Core Model
EF Core uses the familiar DbContext class from prior versions of Entity Framework. myPrayerJournal does take advantage of a feature that just arrived in EF Core 2.1, though - the DbQuery type. DbSets are collections of entities that generally map to an underlying database table. They can be mapped to views, but unless it's an updateable view, updating those entities results in a runtime error; plus, since they can't be updated, there's no need for the change tracking mechanism to care about the entities returned. DbQuery addresses both these concerns, providing lightweight read-only access to data from views.
The DbContext class is defined in Data.fs (mpj:Data.fs), starting in line 189. It's relatively straightforward, though if you have only ever seen a C# model, it's a bit different. The combination of val mutable x : [type] and the [<DefaultValue>] attribute are the F# equivalent of C#'s [type] x; declaration, which creates a variable and initializes reference types to null. The EF Core runtime provides these instances to their setters (lines 203, 206, 209, and 212), and the application code uses them via the getters (a line earlier, each).
The OnModelCreating overridden method (line 214) is called when the runtime first creates its instance of the data model. Within this method, we call the .configureEF function of each of our database types. The name of this function isn't prescribed, and we could define the entire model without even referencing the data types of our entities; however, this technique gives us a "configure where it's defined" paradigm with each entity type. While the EF "Code First" model creates tables that don't need a lot of configuring; sine we're writing a DbContext to target an existing database, we must provide more information about the layout of the database tables.
Let's start out by taking a look at History.configureEF (line 50). Line 53 says that we're going to the table history. This seems to be a no-brainer, but EF Core would (by convention) be expecting a History table; since PostgreSQL uses a different syntax for case-sensitive names, these queries would look like SELECT ... FROM "History" ..., resulting in a nice "relation does not exist" error. Line 54 defines our compound key (requestId and asOf). Lines 55-57 define certain properties of the entity as required; if we try to store an entity where these fields are not set, the runtime will raise an exception before even trying to take it to the database. (F#'s non-nullability makes this a non-issue, but it still needs to be defined to match the database.) Line 58 may seem to do nothing, but what it does is make the text property immediately visible to the model builder; then, we can define an OptionConverter<string>2 for it, which will translate between null and string option (None = null, Some [x] = [x]). (Lines 60-61 are left over from when I was trying to figure out why line 62 was raising an exception, leading to the addition of line 58; they could safely be remove, and will be for a post-1.0 release.)
History is the most complex configuration, but let's take a peek at Request.configureEF (line 126) to see one more interesting technique. Lines 107-110 define the history and notes collections on the Request type; lines 138-145 define the one-to-many relationship (without a foreign key entity in the child types). Note the casts to IEnumerable<x> (lines 138 and 142) and obj (lines 140 and 144); while F# is good about inferring types in a lot of cases, these functions are two places it is not. We can use the :> operator for the cast, because these types are part of the inheritance chain. (The :?> operator is used for potentially unsafe casts.)
Reading and Writing Data
EF Core uses the "unit of work" pattern with its DbContext class. Each instance maintains knowledge of the entities it's loaded, and does change tracking against those entities, so it knows what commands to issue when .SaveChanges() is called. It doesn't do this for free, though, and while EF Core does this much more efficiently than Entity Framework proper, F# record types do not support mutation; given req is a Request instance, { req with showAfter = 123456789L } returns a new Request instance.
This is the problem whose solution is enabled by lines 227-233 in Data.fs. We can manually register an instance of an entity as either added or modified, and when we call .SaveChanges() (or .SaveChangesAsync()), the runtime will generate the SQL to update the data store accordingly. This also allows us to use .AsNoTracking() in our queries (lines 250, 258, 265, and 275), which means that the resultant entities will not be registered with the change tracker (which saves overhead). Notice that we don't specify that on line 243; since Journal is defined as a DbQuery instead of a DbSet, we get change-tracking-avoidance for free.
Generally speaking, the perferred method of writing queries against a DbContext instance is to define extension methods against it. These are static by default, and they enable the context to be as lightweight as possible, while extending it when necessary. However, since this context is so small, we've created 6 methods on the context that we use to obtain data.
If you've been reading along with the tour, we have already seen a few API handler functions (mpj:Handlers.fs) that use the data context. Line 137 has the handler for /api/journal, the endpoint to retrieve a user's active requests. It uses .JournalByUserId(), defined in Data.fs line 242, whose signature is string -> JournalRequest seq. (The latter is an F# alias for IEnumerable<JournalRequest>.) Back in the handler, we use db ctx to get the context (more on that below), then call the method; we're piping the output of userId ctx into it, so it gets its lone parameter from the pipe, then its output is piped to the asJson function we discussed as part of the API.
Line 192, the handler for /api/request/[id]/history, demonstrates both inserting and updating data. We attempt to retrieve the request by its ID and the user ID; if that fails, we return a 404. If it succeeds, though, we add a history entry (lines 201-207), and optionally update the showAfter field of the request based on its recurrence. Finally, the call on line 212 commits the changes for this particular instance. Since the .SaveChanges[Async]() methods return the number of records affected, we cannot use the do! operator for this; F# makes you explicitly ignore values you aren't either returning or assigning to a name. However, defining _ as a parameter or name demonstrates that we realize there is a value to be had, we just are not going to do anything with it.
Getting a DbContext
Since Giraffe sits atop ASP.NET Core, we use the same technique; we use the .AddDbContext() extension method on the IServiceCollection interface, and assign it when we set up the dependency injection container. In our case, it's in Program.fs (mpj:Program.fs) line 50, where we also direct it to use a PostgreSQL connection defined by the connection string "mpj". (This comes from the unified configuration built from appsettings.json and appsettings.[Environment].json.) If we look back at Handlers.fs, lines 45-47, we see the definition of the db ctx call we used earlier. We're using the Giraffe-provided GetService<'T>() extension method to return this instance.
Our tour is nearing its end, but we still have a few stops to go. Next time, we'll look at how we generated documentation to tell people how to use this app.
1 Writing this post has shown me that I need to either create a SQL creation script for the repo, or create an EF Core initial migration/model, so the database ever has to be recreated from scratch. It's good to write about things after you do them!
2 This is also a package I wrote; it's available on NuGet, and I also wrote a post about what it does.