Friday, June 5, 2015

Create a Data Access Layer with SQLite.swift

To use sqlite.swift with Swift 2, see the Create a Data Access Layer with SQLite.Swift and Swift 2 post.

SQLite is an open source, lightweight and cross platform relational database however it does require good knowledge of SQL to use.  For me that is not much of a problem however it is always better if we can avoid embedding SQL statements in our source code.  This is where frameworks like SQLite.swift come in.  SQLite.swift is a type-safe, Swift language layer over SQLite3 that allows us to access an SQLite database in a pure Swift type interface. 
It is important, when designing an application, to design a good data access layer between our application and its backend data storage.  While some may argue that is what Core Data is for, I am just not a big fan of it especially if we are planning on porting our applications to other platforms.  In this blog post, I will show how we can use the SQLite.swift framework to design a good data access layer for our applications written in Swift.
We can download sqlite.swift from it’s githubrepository.  There are good instructions on how to install it either though CocoaPods or manually in our applications here.  Once we have sqlite.swift setup within our application we will be ready to design our data access layer.
In this blog post, we will mainly be discussing how to design the data access layer.  We will back up our design with working example code but the focus of the post will be on the design and the code is there to reinforce that design.

Data Access Layer Introduction


Our data access layer will consist of three layers.  The bottom most, connection, layer will consist of one class named SQLiteDataStore which will contain the connection handle for our SQLite database.  This class will implement the singleton pattern so all access to the database will go though one connection handle.
The next layer will be a data helper layer that will contain one class for each of our tables.  These data helper classes will contain methods to create, insert, delete and query a specific table.  We will want to create a protocol for the data helper classes to ensure that they contain a minimum set of functionality like creating the table.
The final layer is the model layer.  This layer will contain classes that model our database tables and will be used to write or retrieve data from the data store.
In our example, we will have two tables.  These are the Teams and the Players table.  From our description above, we can probably guess that this means we will need two data helper classes (TeamDataHelper and PlayerDataHelper) and two model classes (Team and Player).  The classes required for our data access layer will look like this:



Connection Layer


Now lets look at the code.  We will begin with the SQLiteDataStore class that will contain the connection handle for our database.  The SQLiteDataStore class will contain the following code:

import Foundation
import SQLite

class SQLiteDataStore {
    static let sharedInstance = SQLiteDataStore()
    let BBDB: Database
   
    private init() {
       
        var path = "BaseballDB.sqlite"
       
        if let dirs : [String] = NSSearchPathForDirectoriesInDomains(NSSearchPathDirectory.DocumentDirectory, NSSearchPathDomainMask.AllDomainsMask, true) as? [String] {
            let dir = dirs[0]
            path = dir.stringByAppendingPathComponent("BaseballDB.sqlite");
            println(path)
        }
       
        BBDB = Database(path)
    }
   
    func createTables() {
        TeamDataHelper.createTable()
        PlayerDataHelper.createTable()
       
    }
}

The SQLiteDataStore class implements the singleton pattern so there is only one instance of the class for the lifecycle of our application.  We implement this pattern by creating a private initiator which is accessed from a static constant named sharedInstance.  Inside the initiator, we set the path to the file that will contain our database and then create the BBDB Database instance with that path.
The SQLiteDataStore class contains a second method named createTables.  The createTables method calls the createTable methods from our data helper classes.  This method will let us create all of the tables from a single method.

Model Layer


Now lets look at the model layer.  The classes in the model layer will mirror the tables in the data store so we can use them to read from or write to the data store.  In our example we will have two classes in the model layer, these are the Player and Team classes.  We will look at the Team class first:

class Team {
   
    var teamId: Int64?
    var city: String?
    var nickName: String?
    var abbreviation: String?
   
    init(teamId: Int64, city: String, nickName: String, abbreviation: String) {
        
        self.teamId = teamId
        self.city = city
        self.nickName = nickName
        self.abbreviation = abbreviation
    }
}

The Team class contains four properties and an init method that will set the properties.  The teamId property is designed to be the unique identifier for the table while the rest of the properties are information about the team.
Now lets see the Player class:

class Player {
   
    var playerId: Int64?
    var firstName: String?
    var lastName: String?
    var number: Int?
    var teamId: Int64?
    var position: Positions?
   
    init (playerId: Int64, firstName: String, lastName: String, number: Int, teamId: Int64, position: Positions?) {
        self.playerId = playerId
        self.firstName = firstName
        self.lastName = lastName
        self.number = number
        self.teamId = teamId
        self.position = position
    }
}

The Player class has five properties and an init method that will set the properties.    The playerId property is designed to be the unique identifier for the table while the rest of the properties will be information about the player.  The final property named position is of the type Positions.  The Positions type is an enum that looks like this.

enum Positions: String {
    case Pitcher = "Pitcher"
    case Catcher = "Catcher"
    case FirstBase = "First Base"
    case SecondBase = "Second Base"
    case ThirdBase = "Third Base"
    case Shortstop = "Shortstop"
    case LeftField = "Left Field"
    case CenterField = "Center Field"
    case RightField = "Right field"
    case DesignatedHitter = "Designated Hitter"
}

This enum is used to define the position that a player plays.

Data Helper Layer


Now lets look at the data helper layer.  This layer will be the bridge to our connection layer and will contain one data helper class per table in our database.  We will begin by creating a data helper protocol that will define the set of methods that each data helper class needs to implement.  The DataHelperProtocol protocol looks like this:

protocol DataHelperProtocol {
    typealias T
    static func createTable() -> Void
    static func insert(item: T) -> Int64
    static func delete(item: T) -> Void
    static func findAll() -> [T]?
}

Within this protocol the four methods that we are defining are:    
createTable:  Creates the table
insert:  insets a row into the table
delete:  deletes a row from the table
findAll:  returns all rows in the table

Notice that we only define one method to query our data stores.  We do this because the methods to query each individual table could be different therefore the method(s) needed to query the tables could be different.  We would need to evaluate the query methods needed for each table on a table by table basis.
Now lets look at the TeamDataHelper class that will confirm to the DataHelperProtocol.  This class will be used to read and write information to the Teams table of the SQLite data store.

import Foundation
import SQLite

class TeamDataHelper: DataHelperProtocol {
    static let TABLE_NAME = "Teams"
   
    static let teamId = Expression<Int64>("teamid")
    static let city = Expression<String>("city")
    static let nickName = Expression<String>("nickname")
    static let abbreviation = Expression<String>("abbreviation")
   
    static let table = SQLiteDataStore.sharedInstance.BBDB[TABLE_NAME]
   
    typealias T = Team
   
    static func createTable() {
        let results = SQLiteDataStore.sharedInstance.BBDB.create(table: table, ifNotExists: true) { t in
            t.column(teamId, primaryKey: true)
            t.column(city)
            t.column(nickName)
            t.column(abbreviation)
        }
    }
   
    static func insert(item: T) -> Int64 {
        if (item.city != nil && item.nickName != nil && item.abbreviation != nil) {
            if let results = table.insert(city <- item.city!, nickName <- item.nickName!, abbreviation <- item.abbreviation!).rowid {
                return results
            }
        }
        return -1

    }
   
    static func delete (item: T) -> Void {
        if let id = item.teamId {
            let query = table.filter(teamId == id)
            query.delete()
        }
    }
   
    static func find(id: Int64) -> T? {
        let query = table.filter(teamId == id)
        var results: T?
        if let item = query.first {
            results = Team(teamId: item[teamId], city: item[city], nickName: item[nickName], abbreviation: item[abbreviation])
        }
        return results
    }
   
    static func findAll() -> [T]? {
        var retArray = [T]()
        for item in table {
            retArray.append(Team(teamId: item[teamId], city: item[city], nickName: item[nickName], abbreviation: item[abbreviation]))
        }
        return retArray
    }
}

We start the TeamDataHelper class by setting the TABLE_NAME property.  This property defines the name for the table within our database.  The next four properties (teamId, city, nickname and abbreviation) define the name and type of each row in the table.  We then set the table property which will be used to access the Team table within the SQLite data store.
Finally we implement each of the four methods defined in the DataHelperProtocol protocol plus one extra method which will search the table by its unique identifier.
The PlayerDataHelper class is similar to the TeamDataHelper class except it is used to read and write to the Players table of the SQLite data store.  The PlayerDataHelper class looks like this:

import Foundation
import SQLite

class PlayerDataHelper: DataHelperProtocol {
    static let TABLE_NAME = "Players"
   
    static let playerId = Expression<Int64>("playerid")
    static let firstName = Expression<String>("firstName")
    static let lastName = Expression<String>("lastName")
    static let number = Expression<Int>("number")
    static let teamId = Expression<Int64>("teamid")
    static let position = Expression<String>("position")

   
    static let table = SQLiteDataStore.sharedInstance.BBDB[TABLE_NAME]
   
    typealias T = Player
   
    static func createTable() {
        let results = SQLiteDataStore.sharedInstance.BBDB.create(table: table, ifNotExists: true) { t in
            t.column(playerId, primaryKey: true)
            t.column(firstName)
            t.column(lastName)
            t.column(number)
            t.column(teamId)
            t.column(position)
       
        }
    }
   
    static func insert(item: T) -> Int64 {
        if (item.firstName != nil && item.lastName != nil && item.teamId != nil && item.position != nil) {
            if let results = table.insert(firstName <- item.firstName!, number <- item.number!, lastName <- item.lastName!, teamId <- item.teamId!, position <- item.position!.rawValue).rowid {
                return results
            }
        }
        return -1
       
    }
   
    static func delete (item: T) -> Void {
        if let id = item.playerId {
            let query = table.filter(playerId == id)
            query.delete()
        }
    }
   
    static func find(id: Int64) -> T? {
        let query = table.filter(playerId == id)
        var results: T?
        if let item = query.first {
            results = Player(playerId: item[playerId], firstName: item[firstName], lastName: item[lastName], number: item[number], teamId: item[teamId], position: Positions(rawValue: item[position]))
        }
        return results
    }
   
    static func findAll() -> [T]? {
        var retArray = [T]()
        for item in table {
            retArray.append(Player(playerId: item[playerId], firstName: item[firstName], lastName: item[lastName], number: item[number], teamId: item[teamId], position: Positions(rawValue: item[position])))
        }
        return retArray
    }
}

We start the PlayerDataHelper class by setting the TABLE_NAME property.  This property defines the name for the table within our database.  The next six properties (playerId, firstName, lastName,  number, teamId and position) define the name and type of each row in the table.  We then set the table property that will be used to access the Player table within the SQLite data store.
Within these data helper classes we can implement any other query methods that are needed for our application. 

Using the Data Access Layer


Now lets see how we would use this data access layer to read and write information to the data store.  To insert a row into a table, all we need to do is to create an instance of the appropriate model class (Player or Team) and then pass it to the appropriate data helper class (PlayerDataHelper or TeamDataHelper).  As an example, here is how we would create four rows in the Teams table:

let bosId = TeamDataHelper.insert(Team(teamId: 0, city: "Boston", nickName: "Red Sox", abbreviation: "BOS"))

let baltId = TeamDataHelper.insert(Team(teamId: 0, city: "Baltimore", nickName: "Orioles", abbreviation: "BAL"))

let tampId = TeamDataHelper.insert(Team(teamId: 0, city: "Tampa Bay", nickName: "Rays", abbreviation: "TB"))

let torId = TeamDataHelper.insert(Team(teamId: 0, city: "Toronto", nickName: "Blue Jays", abbreviation: "TOR"))

Here is how we would create three rows in the Players table:

let ortizId = PlayerDataHelper.insert(Player(playerId: 0, firstName: "David", lastName: "Ortiz", number: 34, teamId: bosId, position: Positions.DesignatedHitter))

let napId = PlayerDataHelper.insert(Player(playerId: 0, firstName: "Mike", lastName: "Napoli", number: 12, teamId: bosId, position: Positions.FirstBase))

let pedId = PlayerDataHelper.insert(Player(playerId: 0, firstName: "Dustin", lastName: "Pedroia", number: 15, teamId: bosId, position: Positions.SecondBase))

To query the database we would call the appropriate method with the data helper class.  The following code would return a list of all teams in the Teams table and prints out the city and nickname for each team.

if let teams = TeamDataHelper.findAll() {
    for team in teams {
        println("\(team.city!) \(team.nickName!)")
    }
}

Conclusion

In this blog post we showed how we could use sqlite.swift and good development practices to create a data access layer that simplifies access to the data and hides the complexity of the underlying data store.  The example shown gives us a good abstraction layer that separates the backend data store from our main application. 
The one thing that our example is missing is a way to query the tables by different elements of the table.  For example, we do not have a way to query all players by team or position.  There are a number of ways that we could implement these queries depending on the complexity needed.  We could create a method for each type of query needed if there are not too many or we could create a single method that will query by each element set in the model object.  How you implement these queries is up to you and really depends on the complexity needed in your application.

In this post I describe how to create a data access layer with SQLite.swift.  In my new book titled Protocol Oriented Programming with Swift I show not only how to create this data access layer with SQLite.swift and Swift 2 but I also show how to use the Bridge pattern to integrate this layer with your application.  You can also read about Protocol-Oriented programming in my POP and OOP blog post.

8 comments:

  1. Really helpful stuff!!! Thank you so much.

    I have a couple of questions for you!
    I'm a newby at Swift/Xcode and so forgive me if any are are silly...

    For the Table definitions, why not use a Struct and get the member-wise initializer for free??
    Oh, I guess you need a reference type to avoid duplicate entries if you were to pass it as an argument....got it.

    Is it possible to use Generics and the new Protocol extensions (Swift 2 with "self") to add the "helper" functions directly to the table structs.

    In other words, could you automatically inherit the insert, delete, findall funcs so the syntax could chain like so:
    Player(...new vals...).insert() and the self type passed to insert would be generic??

    I'll keep reading & exploring your cool code....

    ReplyDelete
    Replies
    1. Check out this post for the table definitions: http://masteringswift.blogspot.com/2015/06/use-tuple-types-to-model-data.html It is a much better way to do it.
      You are also spot on with using Swift 2 protocol definitions. Once Swift 2 comes out and sqlite.swift finalizes the Swift 2 support I will create another post that shows how to do this with Swift 2.
      I am glad you find the posts helpful.

      Delete
  2. I tried to use the new version of sqlite.swift, but unfortunately a lot of properties/methods are not supported anymore, like query.first. Perhaps you can update your post using the newer version with swift 2 support.
    thanks for your posting
    arnold

    ReplyDelete
    Replies
    1. Wanted to let you know that I just finished the post using Swift 2. You can find it here: http://masteringswift.blogspot.com/2015/09/create-data-access-layer-with.html

      Delete
  3. Thanks for the comment, I am planning on doing another post as soon but currently I am tied up with updating my Mastering Swift book for Swift 2. I am hoping to have the time to update the post within the next two weeks.

    ReplyDelete
  4. Programming is very interesting and creative thing if you do it with love. Your blog code helps a lot to beginners to learn programming from basic to advance level. I really love this blog because I learn a lot from here and this process is still continuing.
    Love from Pro Programmer

    ReplyDelete
  5. Hi, this is awesome. I have one doubt, how to drop the all tables from database(Clear the database)?.

    ReplyDelete