How to pass input variable to SQL statement in R shiny? - r

Question

Usually, variables can be passed to SQL statements using paste. Interestingly this doesn't work with input variables in R shiny. Using the below code I get the following error message. How can I solve this?
Error in .getReactiveEnvironment()$currentContext() :
Operation not allowed without an active reactive context. (You tried to do
something that can only be done from inside a reactive expression or observer.)
--ui.R--
shinyUI(bootstrapPage(
selectInput(inputId = "segment",
label = "segment",
choices = c(1, 2, 3, 4),
selected = 1),
plotOutput(outputId = "main_plot", height = "300px")
))
--server.R--
shinyServer(function(input, output) {
database <- dbConnect(MySQL(), group= "zugangsdaten", dbname= 'database')
input<- input$segment
table <- dbGetQuery(database, statement =
paste("
SELECT a,b FROM table1
WHERE id = ",input,"
AND created_at>='2015-08-01'
"))
output$main_plot <- renderPlot({
plot(a,b)
})
})

Answer 1

The data query needs to be evaluated in a reactive context.
One way would be to move the data query itself into the renderPlot() context e.g.
--server.R--
shinyServer(function(input, output) {
database <- dbConnect(MySQL(), group= "zugangsdaten", dbname= 'database')
output$main_plot <- renderPlot({
table <- dbGetQuery(database, statement =
paste("
SELECT a,b FROM table1
WHERE id = ",input$segment,"
AND created_at>='2015-08-01'
"))
plot(table$a,table$b)
})
})
However, it's better to construct a reactive conductor for the data which can be evaluated once when any updates happen and re-used in multiple reactive end-points (see here for details).
This would look something like:
--server.R--
shinyServer(function(input, output) {
database <- dbConnect(MySQL(), group= "zugangsdaten", dbname= 'database')
table <- reactive({
dbGetQuery(database, statement =
paste("
SELECT a,b FROM table1
WHERE id = ",input$segment,"
AND created_at>='2015-08-01'
")
)
})
output$main_plot <- renderPlot({
plot(table()$a,table()$b)
})
})

Answer 2

For flexibility you can also use sub function to substitute part of the query string, this is quite clean approach
table <- reactive({
my_query <- 'SELECT a,b FROM table1 WHERE id = SOMETHING AND created_at >= 2015-08-01'
my_query <- sub("SOMETHING",input$segment,my_query)
dbGetQuery(database,noquote(my_query))
})