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from django.db.models import Func, Transform, Value, fields
from django.db.models.functions import Coalesce
class ConcatPair(Func):
"""
Concatenate two arguments together. This is used by `Concat` because not
all backend databases support more than two arguments.
"""
function = 'CONCAT'
def as_sqlite(self, compiler, connection):
coalesced = self.coalesce()
return super(ConcatPair, coalesced).as_sql(
compiler, connection, template='%(expressions)s', arg_joiner=' || '
)
def as_mysql(self, compiler, connection):
# Use CONCAT_WS with an empty separator so that NULLs are ignored.
return super().as_sql(
compiler, connection, function='CONCAT_WS', template="%(function)s('', %(expressions)s)"
)
def coalesce(self):
# null on either side results in null for expression, wrap with coalesce
c = self.copy()
expressions = [
Coalesce(expression, Value('')) for expression in c.get_source_expressions()
]
c.set_source_expressions(expressions)
return c
class Concat(Func):
"""
Concatenate text fields together. Backends that result in an entire
null expression when any arguments are null will wrap each argument in
coalesce functions to ensure a non-null result.
"""
function = None
template = "%(expressions)s"
def __init__(self, *expressions, **extra):
if len(expressions) < 2:
raise ValueError('Concat must take at least two expressions')
paired = self._paired(expressions)
super().__init__(paired, **extra)
def _paired(self, expressions):
# wrap pairs of expressions in successive concat functions
# exp = [a, b, c, d]
# -> ConcatPair(a, ConcatPair(b, ConcatPair(c, d))))
if len(expressions) == 2:
return ConcatPair(*expressions)
return ConcatPair(expressions[0], self._paired(expressions[1:]))
class Length(Transform):
"""Return the number of characters in the expression."""
function = 'LENGTH'
lookup_name = 'length'
output_field = fields.IntegerField()
def as_mysql(self, compiler, connection):
return super().as_sql(compiler, connection, function='CHAR_LENGTH')
class Lower(Transform):
function = 'LOWER'
lookup_name = 'lower'
class StrIndex(Func):
"""
Return a positive integer corresponding to the 1-indexed position of the
first occurrence of a substring inside another string, or 0 if the
substring is not found.
"""
function = 'INSTR'
arity = 2
output_field = fields.IntegerField()
def as_postgresql(self, compiler, connection):
return super().as_sql(compiler, connection, function='STRPOS')
class Substr(Func):
function = 'SUBSTRING'
def __init__(self, expression, pos, length=None, **extra):
"""
expression: the name of a field, or an expression returning a string
pos: an integer > 0, or an expression returning an integer
length: an optional number of characters to return
"""
if not hasattr(pos, 'resolve_expression'):
if pos < 1:
raise ValueError("'pos' must be greater than 0")
expressions = [expression, pos]
if length is not None:
expressions.append(length)
super().__init__(*expressions, **extra)
def as_sqlite(self, compiler, connection):
return super().as_sql(compiler, connection, function='SUBSTR')
def as_oracle(self, compiler, connection):
return super().as_sql(compiler, connection, function='SUBSTR')
class Upper(Transform):
function = 'UPPER'
lookup_name = 'upper'
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