Question

I am working on access a database using F# and my initial attempt at creating a function to create the update query is flawed.

let BuildUserUpdateQuery (oldUser:UserType) (newUser:UserType) =
    let buf = new System.Text.StringBuilder("UPDATE users SET ");
    if (oldUser.FirstName.Equals(newUser.FirstName) = false)  then buf.Append("SET first_name='").Append(newUser.FirstName).Append("'" ) |> ignore
    if (oldUser.LastName.Equals(newUser.LastName) = false)  then buf.Append("SET last_name='").Append(newUser.LastName).Append("'" ) |> ignore
    if (oldUser.UserName.Equals(newUser.UserName) = false)  then buf.Append("SET username='").Append(newUser.UserName).Append("'" ) |> ignore
    buf.Append(" WHERE id=").Append(newUser.Id).ToString()

This doesn’t properly put a , between any update parts after the first, for example:

UPDATE users SET first_name='Firstname', last_name='lastname' WHERE id=...

I could put in a mutable variable to keep track when the first part of the set clause is appended, but that seems wrong.

I could just create an list of tuples, where each tuple is oldtext, newtext, columnname, so that I could then loop through the list and build up the query, but it seems that I should be passing in a StringBuilder to a recursive function, returning back a boolean which is then passed as a parameter to the recursive function.

Does this seem to be the best approach, or is there a better one?

UPDATE:

Here is what I am using as my current solution, as I wanted to make it more generalized, so I just need to write an abstract class for my entities to derive from and they can use the same function. I chose to split up how I do the function so I can pass in how to create the SET part of the update so I can test with different ideas.

let BuildUserUpdateQuery3 (oldUser:UserType) (newUser:UserType) =
    let properties = List.zip3 oldUser.ToSqlValuesList newUser.ToSqlValuesList oldUser.ToSqlColumnList 
    let init = false, new StringBuilder()
    let anyChange, (formatted:StringBuilder) = 
        properties |> Seq.fold (fun (anyChange, sb) (oldVal, newVal, name) ->
            match(oldVal=newVal) with
            | true -> anyChange, sb
            | _ ->
                match(anyChange) with
                | true -> true, sb.AppendFormat(",{0} = '{1}'", name, newVal)
                | _ -> true, sb.AppendFormat("{0} = '{1}'", name, newVal)                    
            ) init
    formatted.ToString()

let BuildUserUpdateQuery (oldUser:UserType) (newUser:UserType) (updatequery:UserType->UserType->String) =
    let buf = StringBuilder("UPDATE users SET ");
    buf.AppendFormat(" {0} WHERE id={1}", (updatequery oldUser newUser), newUser.Id)

let UpdateUser conn (oldUser:UserType) (newUser:UserType) =
    let query = BuildUserUpdateQuery oldUser newUser BuildUserUpdateQuery3
    execNonQuery conn (query.ToString())

Answer 1

Is this the tuple solution you had in mind?

let BuildUserUpdateQuery (oldUser:UserType) (newUser:UserType) =
    let buf = StringBuilder("UPDATE users set ")
    let properties = 
        [(oldUser.FirstName, newUser.FirstName, "first_name")
         (oldUser.LastName, newUser.LastName, "last_name")
         (oldUser.UserName, newUser.UserName, "username")]
         |> Seq.map (fun (oldV, newV, field) -> 
                        if oldV <> newV 
                            then sprintf "%s='%s'" field newV 
                            else null)
         |> Seq.filter (fun p -> p <> null)
         |> Seq.toArray
    if properties.Length = 0
        then None
        else
            bprintf buf "%s" (String.Join(", ", properties))
            bprintf buf " where id=%d" newUser.Id
            Some <| buf.ToString()

I don’t see how the recursive solution could be simpler than this…

BTW I would strongly advise to use proper SQL parameters instead of just concatenating the values, you might become vulnerable to injection attacks…