package caqti

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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 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 be defined directly in a module scope. However, an optional oneshot parameter may be passed to indicate a dynamically generated query.


type query =
  1. | L of string

    Literal code. May contain incomplete fragments.

  2. | P of int

    P i refers to parameter number i, counting from 0.

  3. | S of query list

    S frags is the concatenation of frags.


A representation of a query string to send to a database, abstracting over parameter references and providing nested concatenation to simplify generation. For databases which only support linear parameters (typically denoted "?"), the driver will reshuffle, elide, and duplicate parameters as needed.

type ('a, 'b, +'m) t constraint 'm = [< `Zero | `One | `Many ]

A request specification embedding a query generator, parameter encoder, and row decoder.

  • 'a is the type of the expected parameter bundle.
  • 'b is the type of a returned row.
  • 'm is the possible multiplicities of returned rows.
val create : ?oneshot:bool -> 'a Caqti_type.t -> 'b Caqti_type.t -> 'm Caqti_mult.t -> (Caqti_driver_info.t -> query) -> ('a, 'b, 'm) t

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.

val param_type : ('a, _, _) t -> 'a Caqti_type.t

param_type req is the type of parameter bundles expected by req.

val row_type : (_, 'b, _) t -> 'b Caqti_type.t

row_type req is the type of rows returned by req.

val row_mult : (_, _, 'm) t -> 'm Caqti_mult.t

row_mult req indicates how many rows req may return. This is asserted when constructing the query.

val query_id : ('a, 'b, 'm) t -> int option

If req is a prepared query, then query_id req is Some id for some id which uniquely identifies req, otherwise it is None.

val query : ('a, 'b, 'm) t -> Caqti_driver_info.t -> query

query req is the function which generates the query of this request possibly tailored for the given driver.


In the following functions, queries are written out as plain strings with the following syntax, which is parsed by Caqti into a query object before being passed to drivers.

Parameters are specified as either

  • "?" for linear substitutions (like Sqlite and MariaDB), or
  • "$1", "$2", ... for non-linear substitutions (like PostgreSQL).

Mixing the two styles in the same query string is not permitted. Note that numbering of non-linear parameters is offset by one compared to the P parameters of the query objects defined in the previous section, in order to be consistent with PostgreSQL conventions.

Static references are references to the ?env argument of the functions below, and thus fixed once the query has been constructed. The query parser accepts two forms:

  • "$(<var>)" is substituted by env driver_info "<var>".
  • "$." is a shortcut for "$(.)".

These aid in substituting configurable fragments, like database schemas or table names. The latter form is suggested for qualifying tables, sequences, etc. with the main database schema. It should expand to a schema name followed by a dot, so that the empty string can be returned if the database does not support schemas or no schema is requested by the user.

Apart from the more generic create_p, these function match up with retrieval functions of Caqti_connection_sig.S and Caqti_response_sig.S according to the multiplicity parameter of their types.

val create_p : ?env:(Caqti_driver_info.t -> string -> query) -> ?oneshot:bool -> 'a Caqti_type.t -> 'b Caqti_type.t -> 'm Caqti_mult.t -> (Caqti_driver_info.t -> string) -> ('a, 'b, 'm) t

create_p 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 a query string based on a preliminary form given by f di, where di is the Caqti_driver_info.t of the target driver. The preliminary query string may contain parameter and static references as described in the introduction of this section.

  • parameter oneshot

    For queries generated on-demand or which are otherwise executed only once, pass true do make the query non-prepared. By default queries are prepared, which is suitable for requests defined at the module level.

  • parameter env

    env driver_info key shall provide the value to substitute for a reference to key in the preliminary query string, or raise Not_found to indicate the reference to key is invalid. Not_found will be re-raised as Invalid_argument with additional information to help locate the bug.

val exec : ?env:(Caqti_driver_info.t -> string -> query) -> ?oneshot:bool -> 'a Caqti_type.t -> string -> ('a, unit, [> `Zero ]) t

exec_p arg_type s is a shortcut for create_p arg_type Caqti_type.unit (fun _ -> s).

val find : ?env:(Caqti_driver_info.t -> string -> query) -> ?oneshot:bool -> 'a Caqti_type.t -> 'b Caqti_type.t -> string -> ('a, 'b, [> `One ]) t

find_p arg_type row_type s is a shortcut for create_p arg_type row_type (fun _ -> s).

val find_opt : ?env:(Caqti_driver_info.t -> string -> query) -> ?oneshot:bool -> 'a Caqti_type.t -> 'b Caqti_type.t -> string -> ('a, 'b, [> `Zero | `One ]) t

find_opt_p arg_type row_type s is a shortcut for create_p arg_type row_type Caqti_mult.zero_or_one (fun _ -> s).

val collect : ?env:(Caqti_driver_info.t -> string -> query) -> ?oneshot:bool -> 'a Caqti_type.t -> 'b Caqti_type.t -> string -> ('a, 'b, [> `Zero | `One | `Many ]) t

collect_p arg_type row_type s is a shortcut for create_p arg_type row_type Caqti_mult.many (fun _ -> s).

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))

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 query 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.


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