fragment methods do not scope to parent

Fix a few bugs in the `Fragment#*_fragment` methods. These bugs
manifested themselves in the `Link` header of the `/documents`
endpoints.

* Do not include `parent: parent` in the `where` clauses; the last
  fragment should be the last one in the entire text not in the
  book/chapter/etc.
* Use `> rank` and `< rank` instead of `rank: rank + 1` and `rank: rank
  - 1`; sometimes the next or previous fragment is separated by a
  distance greater than `1`.
* Add missing method tests in `Fragment`.

app/models/fragment.rb

@@ -11,7 +11,7 @@ class Fragment < ApplicationRecord
   validates :descendent_rank, presence: true, numericality: { only_integer: true }
 
   def previous_fragment
-    Fragment.find_by(document: document, parent: parent, level: level, rank: rank - 1)
+    Fragment.where(document: document, level: level, rank: -Float::INFINITY...rank).order(:rank).last
   end
 
   def previous_range(stop_fragment)
@@ -21,7 +21,7 @@ def previous_range(stop_fragment)
   end
 
   def next_fragment
-    Fragment.find_by(document: document, parent: parent, level: level, rank: rank + 1)
+    Fragment.where(document: document, level: level, rank: (rank + 1)..Float::INFINITY).order(:rank).first
   end
 
   def next_range(stop_fragment)
@@ -31,7 +31,7 @@ def next_range(stop_fragment)
   end
 
   def first_fragment
-    Fragment.where(document: document, parent: parent, level: level).order(:rank).first
+    Fragment.where(document: document, level: level).order(:rank).first
   end
 
   def first_range(stop_fragment)
@@ -41,7 +41,7 @@ def first_range(stop_fragment)
   end
 
   def last_fragment
-    Fragment.where(document: document, parent: parent, level: level).order(:rank).last
+    Fragment.where(document: document, level: level).order(:rank).last
   end
 
   def last_range(stop_fragment)

spec/models/fragment_spec.rb

@@ -28,4 +28,216 @@
     it { should validate_uniqueness_of(:ref).scoped_to(:document_id) }
     it { should validate_uniqueness_of(:rank).scoped_to(:document_id) }
   end
+
+  describe 'navigation methods' do
+    let!(:collection) do
+      Collection.create(urn: 'urn', title: 'title', display_type: 'resource', cite_structure: %w[book chapter])
+    end
+    let!(:document) { Document.create(urn: 'urn', xml: '<document/>', collection: collection) }
+
+    let!(:parent1) do
+      Fragment.create(
+        ref: 'p1',
+        xml: '<p1/>',
+        level: 1,
+        rank: 1,
+        descendent_rank: 4,
+        document: document,
+      )
+    end
+    let!(:child11) do
+      Fragment.create(
+        ref: 'c11',
+        xml: '<c11/>',
+        level: 2,
+        rank: 2,
+        descendent_rank: 2,
+        parent: parent1,
+        document: document,
+      )
+    end
+    let!(:child12) do
+      Fragment.create(
+        ref: 'c12',
+        xml: '<c12/>',
+        level: 2,
+        rank: 3,
+        descendent_rank: 3,
+        parent: parent1,
+        document: document,
+      )
+    end
+    let!(:child13) do
+      Fragment.create(
+        ref: 'c13',
+        xml: '<c13/>',
+        level: 2,
+        rank: 4,
+        descendent_rank: 4,
+        parent: parent1,
+        document: document,
+      )
+    end
+
+    let!(:parent2) do
+      Fragment.create(ref: 'p2', xml: '<p2/>', level: 1, rank: 5, descendent_rank: 8, document: document)
+    end
+    let!(:child21) do
+      Fragment.create(
+        ref: 'c21',
+        xml: '<c21/>',
+        level: 2,
+        rank: 6,
+        descendent_rank: 6,
+        parent: parent2,
+        document: document,
+      )
+    end
+    let!(:child22) do
+      Fragment.create(
+        ref: 'c22',
+        xml: '<c22/>',
+        level: 2,
+        rank: 7,
+        descendent_rank: 7,
+        parent: parent2,
+        document: document,
+      )
+    end
+    let!(:child23) do
+      Fragment.create(
+        ref: 'c23',
+        xml: '<c23/>',
+        level: 2,
+        rank: 8,
+        descendent_rank: 8,
+        parent: parent2,
+        document: document,
+      )
+    end
+
+    let!(:parent3) do
+      Fragment.create(ref: 'p3', xml: '<p3/>', level: 1, rank: 9, descendent_rank: 9, document: document)
+    end
+
+    describe '#previous_fragment' do
+      it 'gets the previous passage of a parent fragment' do
+        expect(parent2.previous_fragment).to eq(parent1)
+      end
+
+      it 'returns nil if there is no previous passage' do
+        expect(parent1.previous_fragment).to be_nil
+      end
+
+      it 'gets the previous passage of a child fragment' do
+        expect(child12.previous_fragment).to eq(child11)
+      end
+
+      it 'gets the last fragment of the previous parent' do
+        expect(child21.previous_fragment).to eq(child13)
+      end
+    end
+
+    describe '#previous_range' do
+      it 'gets the previous range of a parent fragment' do
+        expect(parent3.previous_range(parent3)).to eq([parent2, parent2])
+      end
+
+      it 'is empty if there is no previous range' do
+        expect(parent1.previous_range(parent2)).to be_empty
+      end
+
+      it 'gets the previous range of a child fragment' do
+        expect(child21.previous_range(child22)).to eq([child12, child13])
+      end
+
+      it 'squishes the previous range if there are not enough children' do
+        expect(child12.previous_range(child13)).to eq([child11, child11])
+      end
+
+      it 'creates a range that straddles the two parents' do
+        expect(child22.previous_range(child23)).to eq([child13, child21])
+      end
+    end
+
+    describe '#next_fragment' do
+      it 'gets the next passage of a parent fragment' do
+        expect(parent1.next_fragment).to eq(parent2)
+      end
+
+      it 'returns nil if there is no next passage' do
+        expect(parent3.next_fragment).to be_nil
+      end
+
+      it 'gets the next passage of a child fragment' do
+        expect(child12.next_fragment).to eq(child13)
+      end
+
+      it 'gets the first fragment of the next parent' do
+        expect(child13.next_fragment).to eq(child21)
+      end
+    end
+
+    describe '#next_range' do
+      it 'gets the next range of a parent fragment' do
+        expect(parent1.next_range(parent1)).to eq([parent2, parent2])
+      end
+
+      it 'is empty if there is no next range' do
+        expect(parent2.next_range(parent3)).to be_empty
+      end
+
+      it 'gets the next range of a child fragment' do
+        expect(child12.next_range(child13)).to eq([child21, child22])
+      end
+
+      it 'squishes the next range if there are not enough children' do
+        expect(child21.next_range(child22)).to eq([child23, child23])
+      end
+
+      it 'creates a range that straddles the two parents' do
+        expect(child11.next_range(child12)).to eq([child13, child21])
+      end
+    end
+
+    describe '#first_fragment' do
+      it 'gets the first passage for a parent fragment' do
+        expect(parent2.first_fragment).to eq(parent1)
+      end
+
+      it 'gets the first passage for a child fragment' do
+        expect(child21.first_fragment).to eq(child11)
+      end
+    end
+
+    describe '#first_range' do
+      it 'gets the first range for a parent fragment' do
+        expect(parent2.first_range(parent3)).to eq([parent1, parent2])
+      end
+
+      it 'gets the first range for a child fragment' do
+        expect(child21.first_range(child23)).to eq([child11, child13])
+      end
+    end
+
+    describe '#last_fragment' do
+      it 'gets the last passage for a parent fragment' do
+        expect(parent1.last_fragment).to eq(parent3)
+      end
+
+      it 'gets the last passage for a child fragment' do
+        expect(child13.last_fragment).to eq(child23)
+      end
+    end
+
+    describe '#last_range' do
+      it 'gets the last range for a parent fragment' do
+        expect(parent1.last_range(parent2)).to eq([parent2, parent3])
+      end
+
+      it 'gets the last range for a child fragment' do
+        expect(child11.last_range(child13)).to eq([child21, child23])
+      end
+    end
+  end
 end