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= Emacs Db - Key/Values stores for Emacs =
An EmacsLisp interface to key/value stores (Mongo, Postgresql Hstore,
etc..) with a simple default implementation based on EmacsLisp Hashtables.
== The interface ==
The idea behind this is to make an interface for interacting with
simple key/value database stores that is portable across all such
stores. So you can make code once but swap out the database with
relative ease.
The interface includes the following functions:
=== db-make reference ===
Make a DB based on the //reference//.
=== db-get key db ===
Get the value from the //db// with the //key//.
=== db-put key value db ===
Put a new //value// into the //db// with the specified //key//.
Return the //value// as it has been put into the //db//.
=== db-map func db &optional query filter ===
Call //func// for every record in //db// optionally //query// filter.
//query//, if specified, should be a list of query terms.
//func// should take 2 arguments:
{{{
key db-value
}}}
where the DB-VALUE is whatever the //db// has attached to the
specified KEY.
This returns an alist of the KEY and the value the function
returned. If //filter// is [[t]] then only pairs with a value are
returned.
=== db-query db query ===
Do //query// on //db// and return the result.
This is [[db-map]] with an identity function.
== Query language ==
{{{db}}} uses the query language provided by the {{{kv}}} library,
which is implemented as a mapping function test on ever value by the
persistent hashtable implementation.
The language should be translatable to just about any database query
language (Mongo, SQL, etc...).
There are only 3 constructs currently, {{{|}}}, {{{&}}} and {{{=}}}.
An expression could be:
{{{
(= field-name value)
}}}
To select any record where {{{field-name}}} has the {{{value}}}
{{{
(|(= field-name value)(= other-field other-value))
}}}
To select any record where {{{field-name}}} has the {{{value}}}
or {{{other-field}}} has the value {{{other-value}}}
{{{
(&(= field-name value)(= other-field other-value))
}}}
To select any record where {{{field-name}}} has the {{{value}}}
and {{{other-field}}} has the value {{{other-value}}}.
Logical combinations of {{{|}}} and {{{&}}} are also possible.
== Hashtable implementation ==
{{{db}}} comes with a simple implementation which can store any
EmacsLisp object (though alists would most usually be preferred).
To make a {{{db}}} with the hash implementation:
{{{
(db-make
`(db-hash
:filename ,(format "/var/cache/some-file")))
}}}
Obviously, most often you will assign the db to a global variable.
{{{
(defvar my-db
(db-make
`(db-hash
:filename ,(format "/var/cache/some-file"))))
(db-put "001" '(("a" . 10)("b" . 20)) my-db)
(db-put "002" '(("a" . 17)("b" . "hello")("xyz" . "well!")) my-db)
(db-get "002" my-db)
}}}
results in:
{{{
(("a" . 17)("b" . "hello")("xyz" . "well!"))
}}}
=== Testing ===
Hash Db's are tied to filenames so to test them you often have to
manage that persistence:
{{{
(unwind-protect
(let ((mydb (db-make `(db-hash :filename "/tmp/mydb")))
(json
(with-temp-buffer
(insert-file-contents "~/work/elmarmalade/users-mongo.json")
(goto-char (point-min))
(json-read))))
(--each json (db-put (car it) (cdr it) mydb))
(list (db-get 'triss mydb)
(db-get 'nicferrier mydb)))
(delete-file "/tmp/mydb.elc"))
}}}
Note the deleting of the {{{elc}}} file. That's how the hash db is
stored.
Alternately one could use {{{fakir-file}}} (see the fakir package) to
mock the file system. But that's harder than just creating and
throwing away the file.
|
