Basic Examples

JoinMost
uses
Join
to join its argument
{3,4}
with whatever else it encounters, which one may think of as “Most” of the
Join
:
[◼]
JoinMost
[{3,4}][{5,6,7},{8,9,10,11}]
In[]:=
{5,6,7,8,9,10,11,3,4}
Out[]=
JoinMost when used with an association as its argument will override key-value pairs from the sequence of lists it encounters if any of them have the same key:
[◼]
JoinMost
[\[LeftAssociation]"a"0\[RightAssociation]][Association["a"7,"b"9]]
In[]:=
a0,b9
Out[]=

Scope

The operator created by JoinRest using an association can accept sequence of associations and will override key-value pairs if and only it encounters any association in the sequence with the same key:
[◼]
JoinMost
[\[LeftAssociation]"a"0,"b"0\[RightAssociation]][Association["a"7,"b"9],Association["a"-8,"c"3]]
In[]:=
a0,b0,c3
Out[]=
If JoinMost has no argument, it operates as an identity on the data it is given:
[◼]
JoinMost
[][Association["a"7,"b"9]]
In[]:=
a7,b9
Out[]=
If JoinMost has a sequence of associations as its argument, it will use the last value for a key it confronts in that sequence to perform the override:
[◼]
JoinMost
[\[LeftAssociation]"a"0\[RightAssociation],\[LeftAssociation]"a"4,"c"-2\[RightAssociation]][Association["a"7,"b"9],Association["a"-8,"c"3]]
In[]:=
a4,b9,c-2
Out[]=

Applications

JoinRest can be right-composed with
Counts
to create an
Association
that coerces an answer for certain anticipated values:
QueryCounts/*
[◼]
JoinMost
[\[LeftAssociation]"a"Missing["not trusted"]\[RightAssociation]][{"a","b","a","b","a","a","a","b"}]
In[]:=
aMissing[not trusted],b3
Out[]=

Properties and Relations

JoinMost works similarly to
Merge
with a
Last
combiner function:
With{data={"a","b","a","b","a","a","a","b"},default=\[LeftAssociation]"a"0\[RightAssociation]},SameQQueryCounts/*
[◼]
JoinMost
[default][data],Merge[{Query[Counts][data],default},Last]​​​​
In[]:=
True
Out[]=
JoinMost is very similar to the resource function JoinRest. In JoinMost its argument is placed last in the
Join
; in JoinRest its argument is placed first in the
Join
.

Possible Issues

JoinMost, like
Join
, will fail to evaluate if the heads of the expressions to be joined differ from one another:
[◼]
JoinMost
[{1,2,3}][Association["a"2]]
In[]:=
Join
:Incompatible elements in Join[a2,{1,2,3}] cannot be joined.
Join[a2,{1,2,3}]
Out[]=

Neat Examples

Why you should read code carefully before trusting the results :
QueryCounts/*
[◼]
JoinMost
[Association[False200]],#survived&[ExampleData[{"Dataset","Titanic"}]]
In[]:=
Out[]=