## The dbmisc package for using SQLite more conveniently in R

Author: Sebastian Kranz, Ulm University

The package dbmisc contains some helper functions to work with databases, e.g. the functions dbGet, dbInsert, dbUpdate and dbDelete simplify common database operations. One main motivation for dbmisc is to facilitate automatic type conversion between R and the database.

For this purpose you can specify a schema of your database as a simple YAML file. With the schema file you can also easily create or update database tables.

There is further functionality that allows automatic logs of database operations and memoization of fetched values. Basic usage is explaned in this README file.

## Installation

dbmisc is hosted on my own drat-powered R archive. To install it, run the following code:

install.packages("dbmisc",
repos = c("https://skranz-repo.github.io/drat/",getOption("repos")))


### Schema file and creation of database tables

The example schema userdb.yaml specifies a database with just one table user (here is an example with more than one table):

user:
table:
userid: TEXT
email: TEXT
age: INTEGER
female: BOOLEAN
created: DATETIME
descr: TEXT
unique_index:
- userid
index:
- email
- [female, age]
- created

Under the field table, all columns of the table are specified using the variable types of the database. The field unique_index specifies that userid is a unique index column, i.e. no two rows with duplicated userid can exist in the database.

The field index specifies three non-unique indices on the table. The second index [female, age] is an index on two columns.

The following R code generates a new SQLite database from this schema in your current working directory:

schema.file = system.file("examples/dbschema/userdb.yaml", package="dbmisc")
db.dir = getwd()
dbCreateSQLiteFromSchema(schema.file=schema.file, db.dir=db.dir, db.name="userdb.sqlite")


You can take a look at the generated database with some software like https://sqlitestudio.pl.

If you want to update the schema for an existing data base you, can use the argument update=TRUE:

dbCreateSQLiteFromSchema(db=db,schema.file=schema.file,update=TRUE)


Then the existing data is converted to the new schema. Newly added columns will be filled with NA values. If update=FALSE, all existing data is deleted.

Of course, for safety reasons always make a backup of your database, before you modify it in this way.

It could be the case that you already have some data in R, e.g. from a CSV file, for which you want to generate a database table. To avoid typing the whole schema, you can use the little helper function schema.template, which generates a skeleton of the yaml code for the schema and copies it to the clipboard. Consider the following code

df = data.frame(a=1:5,b="hi",c=Sys.Date(),d=Sys.time())
schema.template(df,"mytable")


It copies to your clipboard the following yaml output, which you can the manually adapt

mytable:
table:
a: INTEGER
b: TEXT
c: DATE
d: DATETIME
index:
- a # example index on first column

### Opening a data base connection with a schema

The functions dbGet, dbInsert, dbUpdate and dbDelete allow common database operations that can use a schema file to facilitate type conversion between R and the database. (So far only tested for SQLite).

The easiest way to use a schema file, is to open the connection and assign the schema with a single command:

db = dbConnectSQLiteWithSchema("userdb.sqlite", schema.file)


Alternatively, you could also assign a schema to an existing database connection with the command set.db.schema

db = set.db.schema(db, schema.file=schema.file)


I typically write for my apps a little function to get the database connection, like:

get.userdb = function(db.dir=getwd()) {
db = getOption("userdb.connection")
if (is.null(db)) {
schema.file = system.file("schema/userdb.yaml",package = "shinyUserDB")
db.file = file.path(db.dir, "userdb.sqlite")
db = dbConnectSQLiteWithSchema(db.file,schema.file)
options(userdb.connection=db)
}
db
}


The function opens just a single database connection. If the connection is already open and the function is called again, it retrieves the connection from a global option. (I am not sure whether there is any benefit from pooling more than one connection with SQLite.)

### Inserting data

The following example inserts an entry into our table user:

new_user = list(created=Sys.time(), userid="user1",age=47, female=TRUE, email="test@email.com", gender="female")
dbInsert(db,table="user", new_user)


Recall that the table user has been specified with the following columns

userid: TEXT
email: TEXT
age: INTEGER
female: BOOLEAN
created: DATETIME
descr: TEXT

Our R list differs in certain aspects from the table:

i) the order of fields is not the same as in the database table,

ii) we have not specified the column descr

iii) we have an additional value gender that is not part of the database.

Using the schema, the function dbInsert conveniently corrects for these differences:

i) orders the values in the right order,

ii) it adds a value descrfilled set to NA, and

iii) it removes gender.

This ‘autocorrection’ allows to avoid some boilerplate code when performing database operations.

The function dbInsert also performs some data type conversions that seem not automatically performed by the functions in the DBI interfaces, e.g. it sets the POSIXct variable created to a DATETIME format that can be stored retrieved from the SQLite database (as SQLite does not really have a DATETIME format it is stored as a floating point number).

You can also pass a data frame to dbInsert in order to insert multiple rows at once.

If you set the argument run=FALSE, dbInsert performs no data base action but just returns the SQL statement that would be run:

dbInsert(db,table="user", new_user,run = FALSE)

## [1] "insert into user values (:userid, :email, :age, :female, :created, :descr)"

Note that dbmisc prepares by default parametrized queries to avoid SQL-injections.

### Getting data with dbGet

The function dbGet retrieves data from the database. E.g. the command

dat = dbGet(db,table="user", list(userid="user1"))


returns a data frame with one row in which userid is equal to “user1”. Again types are converted to standard R formats. For example, SQLite stores BOOLEANS internally as INTEGER, but based on the schema, dbGet will correctly convert the variable female to a logical variable in R. Also DATETIME variables will be correctly converted to POSIXct.

If you set the argument run=FALSE, you can get the SQL query that dbGet runs:

dbGet(db,table="user", list(userid="user1"), run = FALSE)

## [1] "SELECT * FROM user WHERE userid = :userid"

The dbGet command allows also for more flexible queries. For example, one can specify selective fields with the argument fields. One can also specify multiple tables joint by the columns specified in joinby:

dbGet(db,c("course","coursestud"), list(courseid="course1"),
fields = "*, coursestud.email", joinby=c("courseid"), run = FALSE)

## [1] "SELECT *, coursestud.email FROM course INNER JOIN coursestud USING(courseid) WHERE courseid = :courseid"

You can also provide a custom SQL command as the argument sql:

sql = "SELECT *, coursestud.email FROM course INNER JOIN coursestud USING(courseid) WHERE courseid = :courseid"

dbGet(db,table=c("course","coursestud"),list(courseid="course1"),sql=sql)


Even for a custom SQL command you can provide one or multiple tables to use the associated schemas when converting data types and parameters that will be used in the parametrized query.

### dbUpdate and dbDelete

The function dbUpdate and dbDelete are similar helper functions to update or delete data sets.

### Logging database modifications

The functions dbInsert, dbUpdate and dbDelete also have an argument log.dir. If provided each call adds an entry to a simple log file that allows to check when some modifications of the database took place.

### Memoisation

The following functionality was useful for some of my shiny apps. The function dbGetMemoise buffers database results in memory. By default, if you call the function again with the same parameters it will get the results from memory.

There is also an argument refetch.if.changed which is by default TRUE if and only if you provide a non-null argument log.dir. When set to TRUE dbGetMemoise will load the data again from the database if the log file has changed.