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Request specification.
A Caqti request is a function to generate a query string from information about the driver, along with type descriptors to encode parameters and decode rows returned from the same query. Requests are passed to Caqti_connection_sig.S.call or one of its shortcut methods provided by a database connection handle.
The request often represent a prepared query, in which case it is static and can be defined directly in a module scope. However, an optional oneshot parameter may be passed to indicate a dynamically generated query.
create arg_type row_type row_mult f is a request which takes parameters of type arg_type, returns rows of type row_type with multiplicity row_mult, and which sends query strings generated from the query f di, where di is the Caqti_driver_info.t of the target driver. The driver is responsible for turning parameter references into a form accepted by the database, while other differences must be handled by f.
parameteroneshot
Disables caching of a prepared statements on connections for this query.
If false (the default), the statement is prepared and a handle is permanently attached to the connection object right before the first time it is executed.
If true, everything allocated in order to execute the statement is released after use.
In other words, the default is suitable for queries which are bound to static modules. Conversely, you should pass ~oneshot:true if the query is dynamically generated, whether it is within a function or a dynamic module, since there will otherwise be a memory leak associated with long-lived connections. You might as well also pass ~oneshot:true if you know that the query will only executed at most once (or a very few times) on each connection.
make_pp_with_param ?env ?driver_info () is a pretty-printer for a request and parameter pair. See make_pp for the optional arguments. This functions is meant for debugging; the output is neither guaranteed to be consistent across releases nor to contain a complete record of the data. Lost database records cannot be reconstructed from the logs.
Due to concerns about exposure of sensitive data in debug logs, this function only prints the parameter values if CAQTI_DEBUG_PARAM is set to true. If you enable it for applications which do not consistenly annotate sensitive parameters with Caqti_type.redacted, make sure your debug logs are well-secured.
How to Dynamically Assemble Queries and Parameters
In some cases, queries are constructed dynamically, e.g. when translating an expression for searching a database into SQL. In such cases the number of parameters and their types will typically vary, as well. A helper like the following can be used to existentially pack the parameter types along with the corresponding parameter values to allow collecing them incrementally:
module Dynparam = struct
type t = Pack : 'a Caqti_type.t * 'a -> t
let empty = Pack (Caqti_type.unit, ())
let add t x (Pack (t', x')) = Pack (Caqti_type.tup2 t' t, (x', x))
end
Now, given a param : Dynparam.t and a corresponding query string qs, one can construct a request and execute it:
let Dynparam.Pack (pt, pv) = param in
let req = Caqti_request.exec ~oneshot:true pt qs in
C.exec req pv
Note that dynamically constructed requests should have ~oneshot:true unless they are memoized. Also note that it is natural to use create for dynamically constructed queries, since it accepts the easily composible Caqti_query.t type instead of plain strings.
This scheme can be specialized for particular use cases, including generation of fragments of the query, which reduces the risk of wrongly matching up parameters with their uses in the query string.