Interface to retrieve record

add validation to the week number field by querying the data from data base and check if the entered week number exist in that , based on conditions you can add validations or please elaborate more.  

you mean do the validation on the week Number field, I have write an expression rule to retrieve the record from database filter via when the weekNum = ri!weekNum.

but I don't know how to call the rule at interface and checking the weeknum based on the user input. 

here is the code for adding validation

{
  a!dateField(
    label: "Date",
    labelPosition: "ABOVE",
    value: ri!date,
    saveInto: {ri!date},
    validations: {if(ri!date>today(),{},"enter a valid date")}
    //if ou have a rule for deciding then
    // validations : if(rule!yourRule(ri!date),{},"not a valid week"
  )
}

I need to check my database for the previous record, if the week num exists in the database, will not allow the user to add the same week num of data into the database.

how to do it?

a!queryEntity(
  entity: cons!TS_TIMESHEET_DS_POINTER,
  query: a!query(
    logicalExpression: a!queryLogicalExpression(
      operator: "AND",
      filters: {
        a!queryFilter(
          field: "workweek",
          operator: "=",
          value: ri!weekNum
        )
      },
      ignoreFiltersWithEmptyValues: true
    ),
    pagingInfo: a!pagingInfo(
      startIndex: 1,
      batchSize: 50
    )
  ),
  fetchTotalCount: true
).data

try this, if this returns true then value exists otherwise it doesn't exist in the data base 

contains(a!queryEntity(
  entity: cons!TS_TIMESHEET_DS_POINTER,
  query: a!query(
    logicalExpression: a!queryLogicalExpression(
      operator: "AND",
      filters: {
        a!queryFilter(
          field: "workweek",
          operator: "=",
          value: ri!weekNum
        )
      },
      ignoreFiltersWithEmptyValues: true
    ),
    pagingInfo: a!pagingInfo(
      startIndex: 1,
      batchSize: 50
    )
  ),
  fetchTotalCount: true
).data."workweek",ri!weekNum)

where should I add this code, in my interface or expression rule

expression rule