diff --git a/docs/ref/models/custom_lookups.txt b/docs/ref/models/custom_lookups.txt new file mode 100644 index 0000000000..6912ca6496 --- /dev/null +++ b/docs/ref/models/custom_lookups.txt @@ -0,0 +1,257 @@ +============== +Custom lookups +============== + +.. module:: django.db.models.lookups + :synopsis: Custom lookups + +.. currentmodule:: django.db.models + +By default Django offers a wide variety of different lookups for filtering +(for example, `exact` and `icontains`). This documentation explains how to +write custom lookups and how to alter the working of existing lookups. In +addition how to transform field values is explained. fFor example how to +extract the year from a DateField. By writing a custom `YearExtract` +transformer it is possible to filter on the transformed value, for example:: + + Author.objects.filter(birthdate__year__lte=1981) + +Currently transformers are only available in filtering. So, it is not possible +to use it in other parts of the ORM, for example this will not work:: + + Author.objects.values_list('birthdate__year') + +A simple Lookup example +~~~~~~~~~~~~~~~~~~~~~~~ + +Lets start with a simple custom lookup. We will write a custom lookup `ne` +which works opposite to `exact`. A `Author.objects.filter(name__ne='Jack')` +will translate to:: + + "author"."name" <> 'Jack' + +A custom lookup will need an implementation and Django needs to be told +the existence of the lookup. The implementation for this lookup will be +simple to write:: + + from django.db.models import Lookup + + class NotEqual(Lookup): + lookup_name = 'ne' + + def as_sql(self, qn, connection): + lhs, lhs_params = self.process_lhs(qn, connection) + rhs, rhs_params = self.process_rhs(qn, connection) + params = lhs_params + rhs_params + return '%s <> %s' % (lhs, rhs), params + +To register the `NotEqual` lookup we will just need to call register_lookup +on the field class we want the lookup to be available:: + + from django.db.models.fields import Field + Field.register_lookup(NotEqual) + +Now Field and all its subclasses have a NotEqual lookup. + +The first notable thing about `NotEqual` is the lookup_name. This name must +be supplied, and it is used by Django in the register_lookup() call so that +Django knows to associate `ne` to the NotEqual implementation. +` +An Lookup works against two values, lhs and rhs. The abbreviations stand for +left-hand side and right-hand side. The lhs is usually a field reference, +but it can be anything implementing the query expression API. The +rhs is the value given by the user. In the example `name__ne=Jack`, the +lhs is reference to Author's name field and Jack is the value. + +The lhs and rhs are turned into values that are possible to use in SQL. +In the example above lhs is turned into "author"."name", [], and rhs is +turned into "%s", ['Jack']. The lhs is just raw string without parameters +but the rhs is turned into a query parameter 'Jack'. + +Finally we combine the lhs and rhs by adding ` <> ` in between of them, +and supply all the parameters for the query. + +A Lookup needs to implement a limited part of query expression API. See +the query expression API for details. + +A simple transformer example +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +We will next write a simple transformer. The transformer will be called +`YearExtract`. It can be used to extract the year part from `DateField`. + +Lets start by writing the implementation:: + + from django.db.models import Extract + + class YearExtract(Extract): + lookup_name = 'year' + output_type = IntegerField() + + def as_sql(self, qn, connection): + lhs, params = qn.compile(self.lhs) + return "EXTRACT(YEAR FROM %s)" % lhs, params + +Next, lets register it for `DateField`:: + + from django.db.models import DateField + DateField.register_lookup(YearExtract) + +Now any DateField in your project will have `year` transformer. For example +the following query:: + + Author.objects.filter(birthdate__year__lte=1981) + +would translate to the following query on PostgreSQL:: + + SELECT ... + FROM "author" + WHERE EXTRACT(YEAR FROM "author"."birthdate") <= 1981 + +An YearExtract class works only against self.lhs. Usually the lhs is +transformed in some way. Further lookups and extracts work against the +transformed value. + +Note the definition of output_type in the `YearExtract`. The output_type is +a field instance. It informs Django that the Extract class transformed the +type of the value to an int. This is currently used only to check which +lookups the extract has. + +The used SQL in this example works on most databases. Check you database +vendor's documentation to see if EXTRACT(year from date) is supported. + +Writing an efficient year__exact lookup +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +When using the above written `year` lookup, the SQL produced will not use +indexes efficiently. We will fix that by writing a custom `exact` lookup +for YearExtract. For example if the user filters on +`birthdate__year__exact=1981`, then we want to produce the following SQL:: + + birthdate >= to_date('1981-01-01') AND birthdate <= to_date('1981-12-31') + +The implementation is:: + + from django.db.models import Lookup + + class YearExact(Lookup): + lookup_name = 'exact' + + def as_sql(self, qn, connection): + lhs, lhs_params = qn.compile(self.lhs.lhs) + rhs, rhs_params = self.process_rhs(qn, connection) + params = lhs_params + rhs_params + lhs_params + rhs_params + return '%s >= to_date(%s || '-01-01') AND %s <= to_date(%s || '-12-31') % (lhs, rhs, lhs, rhs), params + + YearExtract.register_lookup(YearExact) + +There are a couple of notable things going on. First, `YearExact` isn't +calling process_lhs(). Instead it skips and compiles directly the lhs used by +self.lhs. The reason this is done is to skip `YearExtract` from adding the +EXTRACT clause to the query. Referring directly to self.lhs.lhs is safe as +`YearExact` can be accessed only from `year__exact` lookup, that is the lhs +is always `YearExtract`. + +Next, as both the lhs and rhs are used multiple times in the query the params +need to contain lhs_params and rhs_params multiple times. + +The final query does string manipulation directly in the database. The reason +for doing this is that if the self.rhs is something else than a plain integer +value (for exampel a `F()` reference) we can't do the transformations in +Python. + +Writing alternative implemenatations for existing lookups +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Sometimes different database vendors require different SQL for the same +operation. For this example we will rewrite a custom implementation for +MySQL for the NotEqual operator. Instead of `<>` we will be using `!=` +operator. + +There are two ways to do this. The first is to write a subclass with a +as_mysql() method and registering the subclass over the original class:: + + class MySQLNotEqual(NotEqual): + def as_mysql(self, qn, connection): + lhs, lhs_params = self.process_lhs(qn, connection) + rhs, rhs_params = self.process_rhs(qn, connection) + params = lhs_params + rhs_params + return '%s != %s' % (lhs, rhs), params + Field.register_lookup(MySQLNotExact) + +The alternate is to monkey-patch the existing class in place:: + def as_mysql(self, qn, connection): + lhs, lhs_params = self.process_lhs(qn, connection) + rhs, rhs_params = self.process_rhs(qn, connection) + params = lhs_params + rhs_params + return '%s != %s' % (lhs, rhs), params + NotEqual.as_mysql = as_mysql + +The subclass way allows one to override methods of the lookup if needed. The +monkey-patch way allows writing different implementations for the same class +in different locations of the project. + +The way Django knows to call as_mysql() instead of as_sql() is as follows. +When qn.compile(notequal_instance) is called, Django first checks if there +is a method named 'as_%s' % connection.vendor. If that method doesn't exist, +the as_sql() will be called. + +The vendor names for Django's in-built backends are 'sqlite', 'postgresql', +'oracle' and 'mysql'. + +The Lookup API +~~~~~~~~~~~~~~ + +An lookup has attributes lhs and rhs. The lhs is something implementing the +query expression API and the rhs is either a plain value, or something that +needs to be compiled into SQL. Examples of SQL-compiled values include `F()` +references and usage of `QuerySets` as value. + +A lookup needs to define lookup_name as a class level attribute. This is used +when registering lookups. + +A lookup has three public methods. The as_sql(qn, connection) method needs +to produce a query string and parameters used by the query string. The qn has +a method compile() which can be used to compile self.lhs. However usually it +is better to call self.process_lhs(qn, connection) instead, which returns +query string and parameters for the lhs. Similary process_rhs(qn, connection) +returns query string and parameters for the rhs. + +The Query Expression API +~~~~~~~~~~~~~~~~~~~~~~~~ + +A lookup can assume that the lhs responds to the query expression API. +Currently direct field references, aggregates and `Extract` instances respond +to this API. + +.. method:: as_sql(qn, connection) + +Responsible for producing the query string and parameters for the expression. +The qn has a compile() method that can be used to compile other expressions. +The connection is the connection used to execute the query. The +connection.vendor attribute can be used to return different query strings +for different backends. + +Calling expression.as_sql() directly is usually an error - instead +qn.compile(expression) should be used. The qn.compile() method will take +care of calling vendor-specific methods of the expression. + +.. method:: as_vendorname(qn, connection) + +Works like as_sql() method. When an expression is compiled by qn.compile() +Django will first try to call as_vendorname(), where vendorname is the vendor +name of the backend used for executing the query. The vendorname is one of +'postgresql', 'oracle', 'sqlite' or 'mysql' for Django's inbuilt backends. + +.. method:: get_lookup(lookup_name):: + +The get_lookup() method is used to fetch lookups. By default the lookup +is fetched from the expression's output type, but it is possible to override +this method to alter that behaviour. + +.. attribute:: output_type + +The output_type attribute is used by the get_lookup() method to check for +lookups. The output_type should be a field instance. + +Note that this documentation lists only the public methods of the API. diff --git a/docs/ref/models/lookups.txt b/docs/ref/models/lookups.txt deleted file mode 100644 index 1f31d8c6fd..0000000000 --- a/docs/ref/models/lookups.txt +++ /dev/null @@ -1,241 +0,0 @@ -============== -Custom lookups -============== - -.. module:: django.db.models.lookups - :synopsis: Custom lookups - -.. currentmodule:: django.db.models - -Django's ORM works using lookup paths when building query filters and other -query conditions. For example in the query Book.filter(author__age__lte=30) -the part "author__age__lte" is the lookup path. - -The lookup path consist of three different parts. First is the related -lookups. In the author__age__lte example the part author refers to Book's -related model Author. Second part of the lookup path is the field. This is -Author's age field in the example. Finally the lte part is commonly called -just lookup. Both the related lookups part and the final lookup part can -contain multiple parts, for example "author__friends__birthdate__year__lte" -has author, friends as related lookups, birthdate as the field and year, lte -as final lookup part. - -This documentation concentrates on writing custom lookups. By writing custom -lookups it is possible to control how Django interprets the final lookup part. - -Django will fetch a ``Lookup`` class from the final field using the field's -method get_lookup(lookup_name). This method is allowed to do these things: - - 1. Return a Lookup class - 2. Raise a FieldError - 3. Return None - -Returning None is only available during backwards compatibility period. -The interpretation is to use the old way of lookup hadling inside the ORM. - -The Lookup class -~~~~~~~~~~~~~~~~ - -A Lookup operates on two values and produces boolean results. The values -are called lhs and rhs. The lhs is usually a field reference, but it can be -anything implementing the query expression API. The rhs is a value to compare -against. - -The API is as follows: - -.. attribute:: lookup_name - -A string used by Django to distinguish different lookups. For example -'exact'. - -.. method:: __init__(lhs, rhs) - -The lhs is something implementing the query expression API. For example in -author__age__lte=30 the lhs is a Col instance referencing the age field of -author model. The rhs is the value to compare against. It can be Python value -(30 in the example) or SQL reference (produced by using F() or queryset for -example). - -.. attribute:: Lookup.lhs - -The left hand side part of this lookup. You can assume it implements the -query expression interface. - -.. attribute:: Lookup.rhs - -The value to compare against. - -.. method:: Lookup.process_lhs(qn, connection) - -Turns the lhs into query string + params. - -.. method:: Lookup.process_rhs(qn, connection) - -Turns the rhs into query string + params. - -.. method:: Lookup.as_sql(qn, connection) - -This method is used to produce the query string of the Lookup. A typical -implementation is usually something like:: - - def as_sql(self, qn, connection): - lhs, params = self.process_lhs(qn, connection) - rhs, rhs_params = self.process_rhs(qn, connection) - params = lhs_params.extend(rhs_params) - return '%s %s', (lhs, rhs), params - -where the is some query operator. The qn is a callable that -can be used to convert strings to quoted variants (that is, colname to -"colname"). Note that the quotation is *not* safe against SQL injection. - -In addition the qn implements method compile() which can be used to turn -anything with as_sql() method to query string. You should always call -qn.compile(part) instead of part.as_sql(qn, connection) so that 3rd party -backends have ability to customize the produced query string. More of this -later on. - -The connection is the connection the SQL is compiled against. - -In addition the Lookup class has some private methods - that is, implementing -just the above mentioned attributes and methods is not enough, instead you -must subclass Lookup. - -The Extract class -~~~~~~~~~~~~~~~~~ - -An Extract is something that converts a value to another value in the query -string. For example you could have an Extract that procudes modulo 3 of the -given value. In SQL this is something like "author"."age" % 3. - -Extracts are used in nested lookups. The Extract class must implement the -query part interface. - -Extracts should be written by subclassing django.db.models.Extract. - -A simple Lookup example -~~~~~~~~~~~~~~~~~~~~~~~ - -This is how to write a simple mod3 lookup for IntegerField:: - - from django.db.models import Lookup, IntegerField - class Mod3(Lookup): - lookup_name = 'mod3' - - def as_sql(self, qn, connection): - lhs_sql, params = self.process_lhs(qn, connection) - rhs_sql, rhs_params = self.process_rhs(qn, connection) - params.extend(rhs_params) - # We need doulbe-escaping for the %%%% operator. - return '%s %%%% %s' % (lhs_sql, rhs_sql), params - - IntegerField.register_lookup(Div3) - -Now all IntegerFields or subclasses of IntegerField will have -a mod3 lookup. For example you could do Author.objects.filter(age__mod3=2). -This query would return every author whose age % 3 == 2. - -A simple nested lookup example -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -Here is how to write an Extract and a Lookup for IntegerField. The example -lookup can be used similarly as the above mod3 lookup, and in addition it -support nesting lookups:: - - class Mod3Extract(Extract): - lookup_name = 'mod3' - - def as_sql(self, qn, connection): - lhs, lhs_params = qn.compile(self.lhs) - return '%s %%%% 3' % (lhs,), lhs_params - - IntegerField.register_lookup(Mod3Extract) - -Note that if you already added Mod3 for IntegerField in the above -example, now Mod3Extract will override that lookup. - -This lookup can be used like Mod3 lookup, but in addition it supports -nesting, too. The default output type for Extracts is the same type as the -lhs' output_type. So, the Mod3Extract supports all the same lookups as -IntegerField. For example Author.objects.filter(age__mod3__in=[1, 2]) -returns all authors for which age % 3 in (1, 2). - -A more complex nested lookup -~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -We will write a Year lookup that extracts year from date field. This -field will convert the output type of the field - the lhs (or "input") -field is DateField, but output is of type IntegerField.:: - - from django.db.models import IntegerField, DateField - from django.db.models.lookups import Extract - - class YearExtract(Extract): - lookup_name = 'year' - - def as_sql(self, qn, connection): - lhs_sql, params = qn.compile(self.lhs) - # hmmh - this is internal API... - return connection.ops.date_extract_sql('year', lhs_sql), params - - @property - def output_type(self): - return IntegerField() - - DateField.register_lookup(YearExtract) - -Now you could write Author.objects.filter(birthdate__year=1981). This will -produce SQL like 'EXTRACT('year' from "author"."birthdate") = 1981'. The -produces SQL depends on used backend. In addtition you can use any lookup -defined for IntegerField, even div3 if you added that. So, -Authos.objects.filter(birthdate__year__div3=2) will return every author -with birthdate.year % 3 == 2. - -We could go further and add an optimized implementation for exact lookups:: - - from django.db.models.lookups import Lookup - - class YearExtractOptimized(YearExtract): - def get_lookup(self, lookup): - if lookup == 'exact': - return YearExact - return super(YearExtractOptimized, self).get_lookup() - - class YearExact(Lookup): - def as_sql(self, qn, connection): - # We will need to skip the extract part, and instead go - # directly with the originating field, that is self.lhs.lhs - lhs_sql, lhs_params = self.process_lhs(qn, connection, self.lhs.lhs) - rhs_sql, rhs_params = self.process_rhs(qn, connection) - # Note that we must be careful so that we have params in the - # same order as we have the parts in the SQL. - params = [] - params.extend(lhs_params) - params.extend(rhs_params) - params.extend(lhs_params) - params.extend(rhs_params) - # We use PostgreSQL specific SQL here. Note that we must do the - # conversions in SQL instead of in Python to support F() references. - return ("%(lhs)s >= (%(rhs)s || '-01-01')::date " - "AND %(lhs)s <= (%(rhs)s || '-12-31')::date" % - {'lhs': lhs_sql, 'rhs': rhs_sql}, params) - -Note that we used PostgreSQL specific SQL above. What if we want to support -MySQL, too? This can be done by registering a different compiling implementation -for MySQL:: - - from django.db.backends.utils import add_implementation - @add_implementation(YearExact, 'mysql') - def mysql_year_exact(node, qn, connection): - lhs_sql, lhs_params = node.process_lhs(qn, connection, node.lhs.lhs) - rhs_sql, rhs_params = node.process_rhs(qn, connection) - params = [] - params.extend(lhs_params) - params.extend(rhs_params) - params.extend(lhs_params) - params.extend(rhs_params) - return ("%(lhs)s >= str_to_date(concat(%(rhs)s, '-01-01'), '%%%%Y-%%%%m-%%%%d') " - "AND %(lhs)s <= str_to_date(concat(%(rhs)s, '-12-31'), '%%%%Y-%%%%m-%%%%d')" % - {'lhs': lhs_sql, 'rhs': rhs_sql}, params) - -Now, on MySQL instead of calling as_sql() of the YearExact Django will use the -above compile implementation.